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Subject: Fractal Questions and Answers Newsgroups: sci.fractals,news.answers,sci.answers From: shirriff@sprite.cs.berkeley.edu (Ken Shirriff) Date: 26 Sep 1994 22:23:05 GMT Archive-name: fractal-faq Last-modified: September 26, 1994 The international computer network Usenet contains discussions on a variety of topics. The Usenet newsgroup "sci.fractals" is devoted to discussions on fractals. Since many common questions reoccur during the discussions, I have compiled this "Frequently Asked Questions" file, consisting of questions and answers contributed by many participants. This file also lists various pro- grams and papers that can be accessed over the Internet by using "anonymous ftp". This file is not intended as a general introduction to fractals, or a set of rigorous definitions, but rather a useful summary from sci.fractals. As a new feature, the fractal FAQ has some links for use with the World Wide Web. It can be accessed with a program such as xmosaic at http://www.cis.ohio-state.edu/hypertext/faq/usenet/fractal-faq/faq.html . Please let me know if there are more links I should add. The questions which are answered are: Q1: I want to learn about fractals. What should I read first? Q2: What is a fractal? What are some examples of fractals? Q3: What is chaos? Q4a: What is fractal dimension? How is it calculated? Q4b: What is topological dimension? Q5: What is a strange attractor? Q6a: What is the Mandelbrot set? Q6b: How is the Mandelbrot set actually computed? Q6c: Why do you start with z=0? Q6d: What are the bounds of the Mandelbrot set? When does it diverge? Q6e: How can I speed up Mandelbrot set generation? Q6f: What is the area of the Mandelbrot set? Q6g: What can you say about the structure of the Mandelbrot set? Q6h: Is the Mandelbrot set connected? Q7a: What is the difference between the Mandelbrot set and a Julia set? Q7b: What is the connection between the Mandelbrot set and Julia sets? Q7c: How is a Julia set actually computed? Q7d: What are some Julia set facts? Q8a: How does complex arithmetic work? Q8b: How does quaternion arithmetic work? Q9: What is the logistic equation? Q10: What is Feigenbaum's constant? Q11a: What is an iterated function system (IFS)? Q11b: What is the state of fractal compression? Q12a: How can you make a chaotic oscillator? Q12b: What are laboratory demonstrations of chaos? Q13: What are L-systems? *Q14: What is some information on fractal music? Q15: How are fractal mountains generated? Q16: What are plasma clouds? Q17a: Where are the popular periodically-forced Lyapunov fractals described? Q17b: What are Lyapunov exponents? Q17c: How can Lyapunov exponents be calculated? Q18: Where can I get fractal T-shirts and posters? Q19: How can I take photos of fractals? Q20: How can 3-D fractals be generated? Q21a: What is Fractint? Q21b: How does Fractint achieve its speed? Q22: Where can I obtain software packages to generate fractals? Q23a: How does anonymous ftp work? Q23b: What if I can't use ftp to access files? Q24a: Where are fractal pictures archived? Q24b: How do I view fractal pictures from alt.binaries.pictures.fractals? Q25: Where can I obtain fractal papers? Q26: How can I join the BITNET fractal discussion? Q27: What are some general references on fractals and chaos? If you are viewing this file with a newsreaders such as "rn" or "trn", you can search for a particular question by using "g^Q5" (that's lower-case g, up- arrow, Q, and a number) where "5" is the question you wish. Or you may browse forward using <control-G> to search for a Subject: line. This file is normally posted to the Usenet groups sci.fractals, news.answers, and sci.answers about every two weeks. Like most FAQs, the most recent copy of this FAQ can be obtained over the Internet for free by "anonymous ftp" to rtfm.mit.edu [18.181.0.24]; it is in /pub/usenet/news.answers/fractal-faq. I am happy to receive more information to add to this file. Also, let me know if you find mistakes. Please send additions, comments, errors, etc. to Ken Shirriff (email: shirriff@cs.Berkeley.EDU, WWW: file://sprite.berkeley.edu/www/ken.shirriff.html ) This file is Copyright 1993,1994 Ken Shirriff. Permission is given for non- profit distribution of this file, as long as the copyright notice and the list of contributors remain attached. However, I would like to be informed if you distribute this file on other systems, so I have an idea of where it is. Con- tact me for more information on distribution. ------------------------------ Subject: Learning about fractals Q1: I want to learn about fractals. What should I read first? A1: _Chaos_ is a good book to get a general overview and history. _Fractals Everywhere_ is a textbook on fractals that describes what fractals are and how to generate them, but it requires knowing intermediate analysis. _Chaos, Fractals, and Dynamics_ is also a good start. There is a longer book list at the end of this file (see "What are some general references?"). ------------------------------ Subject: What is a fractal? Q2: What is a fractal? What are some examples of fractals? A2: A fractal is a rough or fragmented geometric shape that can be subdivided in parts, each of which is (at least approximately) a reduced-size copy of the whole. Fractals are generally self-similar and independent of scale. There are many mathematical structures that are fractals; e.g. Sierpinski triangle, Koch snowflake, Peano curve, Mandelbrot set, and Lorenz attractor. Fractals also describe many real-world objects, such as clouds, mountains, turbulence, and coastlines, that do not correspond to simple geometric shapes. Benoit Mandelbrot gives a mathematical definition of a fractal as a set for which the Hausdorff Besicovich dimension strictly exceeds the topological di- mension. However, he is not satisfied with this definition as it excludes sets one would consider fractals. According to Mandelbrot, who invented the word: "I coined _fractal_ from the Latin adjective _fractus_. The corresponding Latin verb _frangere_ means "to break:" to create irregular framgents. It is therefore sensible - and how ap- propriate for our needs! - that, in addition to "fragmented" (as in _fraction_ or _refraction_), _fractus_ should also mean "irregular," both meanings being preserved in _fragment_." (_The Fractal Geometry of Nature_, page 4.) ------------------------------ Subject: Chaos Q3: What is chaos? A3: Chaos is apparently unpredictable behavior arising in a deterministic sys- tem because of great sensitivity to initial conditions. Chaos arises in a dynamical system if two arbitrarily close starting points diverge exponential- ly, so that their future behavior is eventually unpredictable. Weather is considered chaotic since arbitrarily small variations in initial conditions can result in radically different weather later. This may limit the possibilities of long-term weather forecasting. (The canonical example is the possibility of a butterfly's sneeze affecting the weather enough to cause a hurricane weeks later.) Devaney defines a function as chaotic if it has sensitive dependence on ini- tial conditions, it is topologically transitive, and periodic points are dense. In other words, it is unpredictable, indecomposable, and yet contains regularity. Allgood and Yorke define chaos as a trajectory that is exponentially unstable and neither periodic or asymptotically periodic. That is, it oscillates ir- regularly without settling down. ------------------------------ Subject: Fractal dimension Q4a: What is fractal dimension? How is it calculated? A4a: A common type of fractal dimension is the Hausdorff-Besicovich Dimension, but there are several different ways of computing fractal dimension. Roughly, fractal dimension can be calculated by taking the limit of the quo- tient of the log change in object size and the log change in measurement scale, as the measurement scale approaches zero. The differences come in what is exactly meant by "object size" and what is meant by "measurement scale" and how to get an average number out of many different parts of a geometrical ob- ject. Fractal dimensions quantify the static *geometry* of an object. For example, consider a straight line. Now blow up the line by a factor of two. The line is now twice as long as before. Log 2 / Log 2 = 1, correspond- ing to dimension 1. Consider a square. Now blow up the square by a factor of two. The square is now 4 times as large as before (i.e. 4 original squares can be placed on the original square). Log 4 / log 2 = 2, corresponding to dimension 2 for the square. Consider a snowflake curve formed by repeatedly replacing ___ with _/\_, where each of the 4 new lines is 1/3 the length of the old line. Blowing up the snowflake curve by a factor of 3 results in a snowflake curve 4 times as large (one of the old snowflake curves can be placed on each of the 4 segments _/\_). Log 4 / log 3 = 1.261... Since the dimension 1.261 is larger than the dimension 1 of the lines making up the curve, the snowflake curve is a fractal. For more information on fractal dimension and scale, access via the WWW http://life.anu.edu.au/complex_systems/tutorial3.html . Fractal dimension references: [1] J. P. Eckmann and D. Ruelle, _Reviews of Modern Physics_ 57, 3 (1985), pp. 617-656. [2] K. J. Falconer, _The Geometry of Fractal Sets_, Cambridge Univ. Press, 1985. [3] T. S. Parker and L. O. Chua, _Practical Numerical Algorithms for Chaotic Systems_, Springer Verlag, 1989. [4] H. Peitgen and D. Saupe, eds., _The Science of Fractal Images_, Springer-Verlag Inc., New York, 1988. ISBN 0-387-96608-0. This book contains many color and black and white photographs, high level math, and several pseudocoded algorithms. [5] G. Procaccia, _Physica D_ 9 (1983), pp. 189-208. [6] J. Theiler, _Physical Review A_ 41 (1990), pp. 3038-3051. References on how to estimate fractal dimension: 1. S. Jaggi, D. A. Quattrochi and N. S. Lam, Implementation and operation of three fractal measurement algorithms for analysis of remote-sensing data., _Computers & Geosciences_ 19, 6 (July 1993), pp. 745-767. 2. E. Peters, _Chaos and Order in the Capital Markets_, New York, 1991. ISBN 0-471-53372-6 Discusses methods of computing fractal dimension. Includes several short programs for nonlinear analysis. 3. J. Theiler, Estimating Fractal Dimension, _Journal of the Optical Society of America A-Optics and Image Science_ 7, 6 (June 1990), pp. 1055-1073. There are some programs available to compute fractal dimension. They are listed in a section below (see "Fractal software"). Q4b: What is topological dimension? A4b: Topological dimension is the "normal" idea of dimension; a point has topological dimension 0, a line has topological dimension 1, a surface has topological dimension 2, etc. For a rigorous definition: A set has topological dimension 0 if every point has arbitrarily small neighborhoods whose boundaries do not intersect the set. A set S has topological dimension k if each point in S has arbitrarily small neighborhoods whose boundaries meet S in a set of dimension k-1, and k is the least nonnegative integer for which this holds. ------------------------------ Subject: Strange attractors Q5: What is a strange attractor? A5: A strange attractor is the limit set of a chaotic trajectory. A strange attractor is an attractor that is topologically distinct from a periodic orbit or a limit cycle. A strange attractor can be considered a fractal attractor. An example of a strange attractor is the Henon attractor. Consider a volume in phase space defined by all the initial conditions a system may have. For a dissipative system, this volume will shrink as the system evolves in time (Liouville's Theorem). If the system is sensitive to initial conditions, the trajectories of the points defining initial conditions will move apart in some directions, closer in others, but there will be a net shrinkage in volume. Ultimately, all points will lie along a fine line of zero volume. This is the strange attractor. All initial points in phase space which ultimately land on the attractor form a Basin of Attraction. A strange attractor results if a system is sensitive to initial conditions and is not conservative. Note: While all chaotic attractors are strange, not all strange attractors are chaotic. Reference: 1. Grebogi, et al., Strange Attractors that are not Chaotic, _Physica D_ 13 (1984), pp. 261-268. ------------------------------ Subject: The Mandelbrot set Q6a: What is the Mandelbrot set? A6a: The Mandelbrot set is the set of all complex c such that iterating z -> z^2+c does not go to infinity (starting with z=0). An image of the Mandelbrot set is available on the WWW at gopher://life.anu.edu.au:70/I9/.WWW/complex_systems/mandel1.gif . Q6b: How is the Mandelbrot set actually computed? A6b: The basic algorithm is: For each pixel c, start with z=0. Repeat z=z^2+c up to N times, exiting if the magnitude of z gets large. If you finish the loop, the point is probably inside the Mandelbrot set. If you exit, the point is outside and can be colored according to how many iterations were completed. You can exit if |z|>2, since if z gets this big it will go to infinity. The maximum number of iterations, N, can be selected as desired, for instance 100. Larger N will give sharper detail but take longer. Q6c: Why do you start with z=0? A6c: Zero is the critical point of z^2+c, that is, a point where d/dz (z^2+c) = 0. If you replace z^2+c with a different function, the starting value will have to be modified. E.g. for z->z^2+z+c, the critical point is given by 2z+1=0, so start with z=-1/2. In some cases, there may be multiple critical values, so they all should be tested. Critical points are important because by a result of Fatou: every attracting cycle for a polynomial or rational function attracts at least one critical point. Thus, testing the critical point shows if there is any stable attractive cycle. See also: 1. M. Frame and J. Robertson, A Generalized Mandelbrot Set and the Role of Critical Points, _Computers and Graphics_ 16, 1 (1992), pp. 35-40. Note that you can precompute the first Mandelbrot iteration by starting with z=c instead of z=0, since 0^2+c=c. Q6d: What are the bounds of the Mandelbrot set? When does it diverge? A6d: The Mandelbrot set lies within |c|<=2. If |z| exceeds 2, the z sequence diverges. Proof: if |z|>2, then |z^2+c| >= |z^2|-|c| > 2|z|-|c|. If |z|>=|c|, then 2|z|-|c| > |z|. So, if |z|>2 and |z|>=c, |z^2+c|>|z|, so the sequence is increasing. (It takes a bit more work to prove it is unbounded and diverges.) Also, note that z1=c, so if |c|>2, the sequence diverges. Q6e: How can I speed up Mandelbrot set generation? A6e: See the information on speed below (see "Fractint"). Also see: 1. R. Rojas, A Tutorial on Efficient Computer Graphic Representations of the Mandelbrot Set, _Computers and Graphics_ 15, 1 (1991), pp. 91-100. Q6f: What is the area of the Mandelbrot set? A6f: Ewing and Schober computed an area estimate using 240,000 terms of the Laurent series. The result is 1.7274... However, the Laurent series converges very slowly, so this is a poor estimate. A project to measure the area via counting pixels on a very dense grid shows an area around 1.5066. (Contact mrob@world.std.com for more information.) Hill and Fisher used distance estimation techniques to rigorously bound the area and found the area is between 1.503 and 1.5701. References: 1. J. H. Ewing and G. Schober, The Area of the Mandelbrot Set, _Numer. Math._ 61 (1992), pp. 59-72. 2. Y. Fisher and J. Hill, Bounding the Area of the Mandelbrot Set, _Numerische Mathematik_, . (Submitted for publication). Available by ftp: legendre.ucsd.edu:/pub/Research/Fischer/area.ps.Z .. Q6g: What can you say about the structure of the Mandelbrot set? A6g: Most of what you could want to know is in Branner's article in _Chaos and Fractals: The Mathematics Behind the Computer Graphics_. Note that the Mandelbrot set in general is _not_ strictly self-similar; the tiny copies of the Mandelbrot set are all slightly different, mainly because of the thin threads connecting them to the main body of the Mandelbrot set. However, the Mandelbrot set is quasi-self-similar. The Mandelbrot set is self-similar under magnification in neighborhoods of Misiurewicz points, however (e.g. -.1011+.9563i). The Mandelbrot set is conjectured to be self- similar around generalized Feigenbaum points (e.g. -1.401155 or -.1528+1.0397i), in the sense of converging to a limit set. References: 1. T. Lei, Similarity between the Mandelbrot set and Julia Sets, _Communications in Mathematical Physics_ 134 (1990), pp. 587-617. 2. J. Milnor, Self-Similarity and Hairiness in the Mandelbrot Set, in _Computers in Geometry and Topology_, M. Tangora (editor), Dekker, New York, pp. 211-257. The "external angles" of the Mandelbrot set (see Douady and Hubbard or brief sketch in "Beauty of Fractals") induce a Fibonacci partition onto it. The boundary of the Mandelbrot set and the Julia set of a generic c in M have Hausdorff dimension 2 and have topological dimension 1. The proof is based on the study of the bifurcation of parabolic periodic points. (Since the boundary has empty interior, the topological dimension is less than 2, and thus is 1.) Reference: 1. M. Shishikura, The Hausdorff Dimension of the Boundary of the Mandelbrot Set and Julia Sets, The paper is available from anonymous ftp: math.sunysb.edu:/preprints/ims91-7.ps.Z [129.49.18.1].. Q6h: Is the Mandelbrot set connected? A6h: The Mandelbrot set is simply connected. This follows from a theorem of Douady and Hubbard that there is a conformal isomorphism from the complement of the Mandelbrot set to the complement of the unit disk. (In other words, all equipotential curves are simple closed curves.) It is conjectured that the Mandelbrot set is locally connected, and thus pathwise connected, but this is currently unproved. Connectedness definitions: Connected: X is connected if there are no proper closed subsets A and B of X such that A union B = X, but A intersect B is empty. I.e. X is connected if it is a single piece. Simply connected: X is simply connected if it is connected and every closed curve in X can be deformed in X to some constant closed curve. I.e. X is simply connected if it has no holes. Locally connected: X is locally connected if for every point p in X, for every open set U containing p, there is an open set V containing p and contained in the connected component of p in U. I.e. X is locally connected if every connected component of every open subset is open in X. Arcwise (or path) connected: X is arcwise connected if every two points in X are joined by an arc in X. (The definitions are from _Encyclopedic Dictionary of Mathematics_.) ------------------------------ Subject: Julia sets Q7a: What is the difference between the Mandelbrot set and a Julia set? A7a: The Mandelbrot set iterates z^2+c with z starting at 0 and varying c. The Julia set iterates z^2+c for fixed c and varying starting z values. That is, the Mandelbrot set is in parameter space (c-plane) while the Julia set is in dynamical or variable space (z-plane). Q7b: What is the connection between the Mandelbrot set and Julia sets? A7b: Each point c in the Mandelbrot set specifies the geometric structure of the corresponding Julia set. If c is in the Mandelbrot set, the Julia set will be connected. If c is not in the Mandelbrot set, the Julia set will be a Cantor dust. You can see an example Julia set on the WWW at gopher://life.anu.edu.au:70/I9/.WWW/complex_systems/julia.gif . Q7c: How is a Julia set actually computed? A7c: The Julia set can be computed by iteration similar to the Mandelbrot computation. The only difference is that the c value is fixed and the initial z value varies. Alternatively, points on the boundary of the Julia set can be computed quickly by using inverse iterations. This technique is particularly useful when the Julia set is a Cantor Set. In inverse iteration, the equation z1 = z0^2+c is reversed to give an equation for z0: z0 = +- sqrt(z1-c). By applying this equation repeatedly, the resulting points quickly converge to the Julia set boundary. (At each step, either the postive or negative root is randomly selected.) This is a nonlinear iterated function system. In pseudocode: z = 1 (or any value) loop if (random number < .5) then z = sqrt(z-c) else z =-sqrt(z-c) endif plot z end loop Q7d: What are some Julia set facts? A7d: The Julia set of any rational map of degree greater than one is perfect (hence in particular uncountable and nonempty), completely invariant, equal to the Julia set of any iterate of the function, and also is the boundary of the basin of attraction of every attractor for the map. Julia set references: 1. A. F. Beardon, _Iteration of Rational Functions : Complex Analytic Dynamical Systems_, Springer-Verlag, New York, 1991. 2. P. Blanchard, Complex Analytic Dynamics on the Riemann Sphere, _Bull. of the Amer. Math. Soc_ 11, 1 (July 1984), pp. 85-141. This article is a detailed discussion of the mathematics of iterated complex functions. It covers most things about Julia sets of rational polynomial functions. ------------------------------ Subject: Complex arithmetic and quaternion arithmetic Q8a: How does complex arithmetic work? A8a: It works mostly like regular algebra with a couple additional formulas: (note: a,b are reals, x,y are complex, i is the square root of -1) Powers of i: i^2 = -1 Addition: (a+i*b)+(c+i*d) = (a+c)+i*(b+d) Multiplication: (a+i*b)*(c+i*d) = a*c-b*d + i*(a*d+b*c) Division: (a+i*b)/(c+i*d) = (a+i*b)*(c-i*d)/(c^2+d^2) Exponentiation: exp(a+i*b) = exp(a)(cos(b)+i*sin(b)) Sine: sin(x) = (exp(i*x)-exp(-i*x))/(2*i) Cosine: cos(x) = (exp(i*x)+exp(-i*x))/2 Magnitude: |a+i*b| = sqrt(a^2+b^2) Log: log(a+i*b) = log(|a+i*b|)+i*arctan(b/a) (Note: log is multivalued.) Log (polar coordinates): log(r*e^(i*theta)) = log(r)+i*theta Complex powers: x^y = exp(y*log(x)) DeMoivre's theorem: x^a = r^a * [cos(a*theta) + i * sin(a*theta)] More details can be found in any complex analysis book. Q8b: How does quaternion arithmetic work? A8b: Quaternions have 4 components (a+ib+jc+kd) compared to the two of complex numbers. Operations such as addition and multiplication can be performed on quaternions, but multiplication is not commutative. Quaternions satisfy the rules i^2=j^2=k^2=-1, ij=-ji=k, jk=-kj=i, ki=-ik=j. ------------------------------ Subject: Logistic equation Q9: What is the logistic equation? A9: It models animal populations. The equation is x -> c*x*(1-x), where x is the population (between 0 and 1) and c is a growth constant. Iteration of this equation yields the period doubling route to chaos. For c between 1 and 3, the population will settle to a fixed value. At 3, the period doubles to 2; one year the population is very high, causing a low population the next year, causing a high population the following year. At 3.45, the period doubles again to 4, meaning the population has a four year cycle. The period keeps doubling, faster and faster, at 3.54, 3.564, 3.569, and so forth. At 3.57, chaos occurs; the population never settles to a fixed period. For most c values between 3.57 and 4, the population is chaotic, but there are also periodic regions. For any fixed period, there is some c value that will yield that period. See "An Introduction to Chaotic Dynamical Systems" for more information. ------------------------------ Subject: Feigenbaum's constant Q10: What is Feigenbaum's constant? A10: In a period doubling cascade, such as the logistic equation, consider the parameter values where period-doubling events occur (e.g. r[1]=3, r[2]=3.45, r[3]=3.54, r[4]=3.564...). Look at the ratio of distances between consecutive doubling parameter values; let delta[n] = (r[n+1]-r[n])/(r[n+2]-r[n+1]). Then the limit as n goes to infinity is Feigenbaum's (delta) constant. Based on independent computations by Jay Hill and Keith Briggs, it has the value 4.669201609102990671853... Note: several books have published incorrect values starting 4.66920166...; the last repeated 6 is a typographical error. The interpretation of the delta constant is as you approach chaos, each periodic region is smaller than the previous by a factor approaching 4.669... Feigenbaum's constant is important because it is the same for any function or system that follows the period-doubling route to chaos and has a one-hump quadratic maximum. For cubic, quartic, etc. there are different Feigenbaum constants. Feigenbaum's alpha constant is not as well known; it has the value 2.502907875095. This constant is the scaling factor between x values at bifurcations. Feigenbaum says, "Asymptotically, the separation of adjacent elements of period-doubled attractors is reduced by a constant value [alpha] from one doubling to the next". If d[n] is the algebraic distance between nearest elements of the attractor cycle of period 2^n, then d[n]/d[n+1] converges to -alpha. References: 1. K. Briggs, How to calculate the Feigenbaum constants on your PC, _Aust. Math. Soc. Gazette_ 16 (1989), p. 89. 2. K. Briggs, A precise calculation of the Feigenbaum constants, _Mathematics of Computation_ 57 (1991), pp. 435-439. 3. K. Briggs, G. R. W. Quispel and C. Thompson, Feigenvalues for Mandelsets, _J. Phys._ A24 (1991), pp. 3363-3368. 4. M. Feigenbaum, The Universal Metric Properties of Nonlinear Transformations, _J. Stat. Phys_ 21 (1979), p. 69. 5. M. Feigenbaum, Universal Behaviour in Nonlinear Systems, _Los Alamos Sci_ 1 (1980), pp. 1-4. Reprinted in _Universality in Chaos_ , compiled by P. Cvitanovic. ------------------------------ Subject: Iterated function systems and compression Q11a: What is an iterated function system (IFS)? A11a: If a fractal is self-similar, you can specify mappings that map the whole onto the parts. Iteration of these mappings will result in convergence to the fractal attractor. An IFS consists of a collection of these (usually affine) mappings. If a fractal can be described by a small number of mappings, the IFS is a very compact description of the fractal. An iterated function system is By taking a point and repeatedly applying these mappings you end up with a collection of points on the fractal. In other words, instead of a single mapping x -> F(x), there is a collection of (usually affine) mappings, and random selection chooses which mapping is used. For instance, the Sierpinski triangle can be decomposed into three self- similar subtriangles. The three contractive mappings from the full triangle onto the subtriangles forms an IFS. These mappings will be of the form "shrink by half and move to the top, left, or right". Iterated function systems can be used to make things such as fractal ferns and trees and are also used in fractal image compression. _Fractals Everywhere_ by Barnsley is mostly about iterated function systems. The simplest algorithm to display an IFS is to pick a starting point, randomly select one of the mappings, apply it to generate a new point, plot the new point, and repeat with the new point. The displayed points will rapidly converge to the attractor of the IFS. An IFS fractal fern can be viewed on the WWW at gopher://life.anu.edu.au:70/I9/.WWW/complex_systems/fern.gif . Q11b: What is the state of fractal compression? A11b: Fractal compression is quite controversial, with some people claiming it doesn't work well, and others claiming it works wonderfully. The basic idea behind fractal image compression is to express the image as an iterated function system (IFS). The image can then be displayed quickly and zooming will generate infinite levels of (synthetic) fractal detail. The problem is how to efficiently generate the IFS from the image. Barnsley, who invented fractal image compression, has a patent on fractal compression techniques (4,941,193). Barnsley's company, Iterated Systems Inc, has a line of products including a Windows viewer, compressor, magnifier program, and hardware assist board. Fractal compression is covered in detail in the comp.compression FAQ file (See "compression-faq"). Ftp: rtfm.mit.edu:/pub/usenet/comp.compression [18.181.0.24]. Two books describing fractal image compression are: 1. M. Barnsley, _Fractals Everywhere_, Academic Press Inc., 1988. ISBN 0- 12-079062-9. This is an excellent text book on fractals. This is probably the best book for learning about the math underpinning fractals. It is also a good source for new fractal types. 2. M. Barnsley and L. Hurd, _Fractal Image Compression_, Jones and Bartlett. ISBN 0-86720-457-5. This book explores the science of the fractal transform in depth. The authors begin with a foundation in information theory and present the technical background for fractal image compression. In so doing, they explain the detailed workings of the fractal transform. Algorithms are illustrated using source code in C. The October 1993 issue of Byte discussed fractal compression. You can ftp sample code: ftp.uu.net:/published/byte/93oct/fractal.exe . An introductory paper is: 1. A. E. Jacquin, Image Coding Based on a Fractal Theory of Iterated Contractive Image Transformation, _IEEE Transactions on Image Processing_, January 1992. A fractal decompression demo program is available by anonymous ftp: lyapunov.ucsd.edu:/pub/inls-ucsd/fractal-2.0 [132.239.86.10]. Another MS-DOS compression demonstration program is available by anonymous ftp: lyapunov.ucsd.edu:/pub/young-fractal . A site with information on fractal compression is legendre.ucsd.edu:/pub/Research/Fisher . On the WWW you can access file://legendre.ucsd.edu/pub/Research/Fisher/fractal.html . Many fractal image compression papers are available from ftp.informatik.uni- freiburg.de:/documents/papers/fractal [132.230.30.80]. A review of the literature is in Guide.ps.Z. ------------------------------ Subject: Chaotic demonstrations Q12a: How can you make a chaotic oscillator? A12a: Two references are: 1. T. S. Parker and L. O. Chua, Chaos: a tutorial for engineers, _Proceedings IEEE_ 75 (1987), pp. 982-1008. 2. _New Scientist_, June 30, 1990, p. 37. Q12b: What are laboratory demonstrations of chaos? A12b: Robert Shaw at UC Santa Cruz experimented with chaos in dripping taps. This is described in: 1. J. P. Crutchfield, Chaos, _Scientific American_ 255, 6 (Dec. 1986), pp. 38-49. 2. I. Stewart, _Does God Play Dice?: the Mathematics of Chaos_, B. Blackwell, New York, 1989. Two references to other laboratory demonstrations are: 1. K. Briggs, Simple Experiments in Chaotic Dynamics, _American Journal of Physics_ 55, 12 (Dec 1987), pp. 1083-1089. 2. J. L. Snider, Simple Demonstration of Coupled Oscillations, _American Journal of Physics_ 56, 3 (Mar 1988), p. 200. ------------------------------ Subject: L-Systems Q13: What are L-systems? A13: A L-system or Lindenmayer system is a formal grammar for generating strings. (That is, it is a collection of rules such as replace X with XYX.) By recursively applying the rules of the L-system to an initial string, a string with fractal structure can be created. Interpreting this string as a set of graphical commands allows the fractal to be displayed. L-systems are very useful for generating realistic plant structures. Some references are: 1. P. Prusinkiewicz and J. Hanan, _Lindenmayer Systems, Fractals, and Plants_, Springer-Verlag, New York, 1989. 2. P. Prusinkiewicz and A. Lindenmayer, _The Algorithmic Beauty of Plants_, Springer-Verlag, NY, 1990. ISBN 0-387-97297-8. A very good book on L-systems, which can be used to model plants in a very realistic fashion. The book contains many pictures. More information can be obtained via the WWW at http://life.anu.edu.au/complex_systems/tutorial2.html and a L-system leaf can be viewed at gopher://life.anu.edu.au:70/I9/.WWW/complex_systems/leaf.gif . ------------------------------ Subject: Fractal music Q14: What is some information on fractal music? A14: One fractal recording is "The Devil's Staircase: Composers and Chaos" on the Soundprint label. Some references, many from an unpublished article by Stephanie Mason, are: 1. R. Bidlack, Chaotic Systems as Simple (But Complex) Compositional Algorithms, _Computer Music Journal_, Fall 1992. 2. C. Dodge, A Musical Fractal, _Computer Music Journal_ 12, 13 (Fall 1988), p. 10. 3. K. J. Hsu and A. Hsu, Fractal Geometry of Music, _Proceedings of the National Academy of Science, USA_ 87 (1990), pp. 938-941. 4. K. J. Hsu and A. Hsu, Self-similatrity of the '1/f noise' called music., _Proceedings of the National Academy of Science USA_ 88 (1991), pp. 3507-3509. 5. C. Pickover, _Mazes for the Mind: Computers and the Unexpected_, St. Martin's Press, New York, 1992. 6. P. Prusinkiewicz, Score Generation with L-Systems, _International Computer Music Conference 86 Proceedings_, 1986, pp. 455-457. 7. _Byte_ 11, 6 (June 1986), pp. 185-196. A IBM-PC program for fractal music is available by ftp to spanky.triumf.ca [142.90.112.1] in [pub.fractals.programs.ibmpc] WTF23.ZIP. A fractal music C++ package is available at http://neural.hampshire.edu:10001/~gzenie/inSanity.html . A WWW page on fractal music is: http://www-ks.rus.uni-stuttgart.de/people/schulz/fmusic . ------------------------------ Subject: Fractal mountains Q15: How are fractal mountains generated? A15: Usually by a method such as taking a triangle, dividing it into 3 subtriangles, and perturbing the center point. This process is then repeated on the subtriangles. This results in a 2-d table of heights, which can then be rendered as a 3-d image. One reference is: 1. M. Ausloos, _Proc. R. Soc. Lond. A_ 400 (1985), pp. 331-350. ------------------------------ Subject: Plasma clouds Q16: What are plasma clouds? A16: They are a Fractint fractal and are similar to fractal mountains. Instead of a 2-d table of heights, the result is a 2-d table of intensities. They are formed by repeatedly subdividing squares. ------------------------------ Subject: Lyapunov fractals Q17a: Where are the popular periodically-forced Lyapunov fractals described? A17a: See: 1. A. K. Dewdney, Leaping into Lyapunov Space, _Scientific American_, Sept. 1991, pp. 178-180. 2. M. Markus and B. Hess, Lyapunov Exponents of the Logistic Map with Periodic Forcing, _Computers and Graphics_ 13, 4 (1989), pp. 553-558. 3. M. Markus, Chaos in Maps with Continuous and Discontinuous Maxima, _Computers in Physics_, Sep/Oct 1990, pp. 481-493. Q17b: What are Lyapunov exponents? A17b: Lyapunov exponents quantify the amount of linear stability or instability of an attractor, or an asymptotically long orbit of a dynamical system. There are as many lyapunov exponents as there are dimensions in the state space of the system, but the largest is usually the most important. Given two initial conditions for a chaotic system, a and b, which are close together, the average values obtained in successive iterations for a and b will differ by an exponentially increasing amount. In other words, the two sets of numbers drift apart exponentially. If this is written e^(n*(lambda)) for n iterations, then e^(lambda) is the factor by which the distance between closely related points becomes stretched or contracted in one iteration. Lambda is the Lyapunov exponent. At least one Lyapunov exponent must be positive in a chaotic system. A simple derivation is available in: 1. H. G. Schuster, _Deterministic Chaos: An Introduction_, Physics Verlag, 1984. Q17c: How can Lyapunov exponents be calculated? A17c: For the common periodic forcing pictures, the lyapunov exponent is: lambda = limit as N->infinity of 1/N times sum from n=1 to N of log2(abs(dx sub n+1 over dx sub n)) In other words, at each point in the sequence, the derivative of the iterated equation is evaluated. The Lyapunov exponent is the average value of the log of the derivative. If the value is negative, the iteration is stable. Note that summing the logs corresponds to multiplying the derivatives; if the product of the derivatives has magnitude < 1, points will get pulled closer together as they go through the iteration. MS-DOS and Unix programs for estimating Lyapunov exponents from short time series are available by ftp: lyapunov.ucsd.edu:/pub/ncsu . Computing Lyapunov exponents in general is more difficult. Some references are: 1. H. D. I. Abarbanel, R. Brown and M. B. Kennel, Lyapunov Exponents in Chaotic Systems: Their importance and their evaluation using observed data, _International Journal of Modern Physics B_ 56, 9 (1991), pp. 1347-1375. 2. A. K. Dewdney, Leaping into Lyapunov Space, _Scientific American_, Sept. 1991, pp. 178-180. 3. M. Frank and T. Stenges, _Journal of Economic Surveys_ 2 (1988), pp. 103- 133. 4. T. S. Parker and L. O. Chua, _Practical Numerical Algorithms for Chaotic Systems_, Springer Verlag, 1989. ------------------------------ Subject: Fractal items Q18: Where can I get fractal T-shirts and posters? A18: One source is Art Matrix, P.O. box 880, Ithaca, New York, 14851, 1-800- PAX-DUTY. Another source is Media Magic; they sell many fractal posters, calendars, videos, software, t-shirts, ties, and a huge variety of books on fractals, chaos, graphics, etc. Media Magic is at PO Box 598 Nicasio, CA 94946, 415-662-2426. A third source is Ultimate Image; they sell fractal t- shirts, posters, gift cards, and stickers. Ultimate Image is at PO Box 7464, Nashua, NH 03060-7464. Another source is Dave Kliman (516)-625-1915, whose products are distributed through Spencer Gifts. ------------------------------ Subject: How can I take photos of fractals? Q19: How can I take photos of fractals? A19: Noel Giffin gets good results with the following setup: Use 100 asa Kodak gold for prints or 64 asa for slides. Use a long lens (100mm) to flatten out the field of view and minimize screen curvature. Use f4 stop. Shutter speed must be longer than frame rate to get a complete image; 1/4 seconds works well. Use a tripod and cable release or timer to get a stable picture. The room should be completely blackened, with no light, to prevent glare and to prevent the monitor from showing up in the picture. You can also obtain high quality images by sending your targa or gif images to a commercial graphics imaging shop. They can provide much higher resolution images. Prices are about $10 for a 35mm slide or negative and about $50 for a high quality 4x5 negative. ------------------------------ Subject: 3-D fractals Q20: How can 3-D fractals be generated? A20: A common source for 3-D fractals is to compute Julia sets with quaternions instead of complex numbers. The resulting Julia set is four dimensional. By taking a slice through the 4-D Julia set (e.g. by fixing one of the coordinates), a 3-D object is obtained. This object can then be displayed using computer graphics techniques such as ray tracing. The papers to read on this are: 1. J. Hart, D. Sandin and L. Kauffman, Ray Tracing Deterministic 3-D Fractals, _SIGGRAPH_, 1989, pp. 289-296. 2. A. Norton, Generation and Display of Geometric Fractals in 3-D, _SIGGRAPH_, 1982, pp. 61-67. 3. A. Norton, Julia Sets in the Quaternions, _Computers and Graphics,_ 13, 2 (1989), pp. 267-278. Two papers on cubic polynomials, which can be used to generate 4-D fractals: 1. B. Branner and J. Hubbard, The iteration of cubic polynomials, part I., _Acta Math_ 66 (1988), pp. 143-206. 2. J. Milnor, Remarks on iterated cubic maps, This paper is available from anonymous ftp: math.sunysb.edu:/preprints/ims90-6.ps.Z . Published in 1991 SIGGRAPH Course Notes #14: Fractal Modeling in 3D Computer Graphics and Imaging. Instead of quaternions, you can of course use other functions. For instance, you could use a map with more than one parameter, which would generate a higher-dimensional fractal. Another way of generating 3-D fractals is to use 3-D iterated function systems (IFS). These are analogous to 2-D IFS, except they generate points in a 3-D space. A third way of generating 3-D fractals is to take a 2-D fractal such as the Mandelbrot set, and convert the pixel values to heights to generate a 3-D "Mandelbrot mountain". This 3-D object can then be rendered with normal computer graphics techniques. ------------------------------ Subject: Fractint Q21a: What is Fractint? A21a: Fractint is a very popular freeware (not public domain) fractal generator. There are DOS, Windows, OS/2, and Unix/X versions. The DOS version is the original version, and is the most up-to-date. There is a new Amiga version. Please note: sci.fractals is not a product support newsgroup for Fractint. Bugs in Fractint/Xfractint should usually go to the authors rather than being posted. Fractint is on many ftp sites. For example: DOS: ftp from wuarchive.wustl.edu:/systems/ibmpc/simtel/graphics [128.252.135.4]. The source is in the file frasr182.zip. The executable is in the file frain182.zip. (The suffix 182 will change as new versions are released.) Fractint is available on Compuserve: GO GRAPHDEV and look for FRAINT.EXE and FRASRC.EXE in LIB 4. There is a collection of map, parameter, etc. files for Fractint, called FracXtra. Ftp from wuarchive.wustl.edu:/systems/ibmpc/simtel/graphics. File is fracxtr5.zip. Windows: ftp to wuarchive.wustl.edu:/systems/ibmpc/simtel/window3 . The source is in the file wins1821.zip. The executable is in the file winf1821.zip. OS/2: available on Compuserve in its GRAPHDEV forum. The files are PM*.ZIP. These files are also available by ftp: ftp-os2.nmsu.edu:/pub/os2/2.0/graphics in pmfra2.zip. Unix: ftp to sprite.berkeley.edu [128.32.150.27]. The source is in the file xfract203.shar.Z. Note: sprite is an unreliable machine; if you can't connect to it, try again in a few hours, or try hijack.berkeley.edu. Xfractint is also available in LIB 4 of Compuserve's GO GRAPHDEV forum in XFRACT.ZIP. Macintosh: there is no Macintosh version of Fractint, although there are several people working on a port. It is possible to run Fractint on the Macintosh if you use Insignia Software's SoftAT, which is a PC AT emulator. Amiga: There is an Amiga version at wuarchive.wustl.edu:/pub/aminet/gfx/fract . For European users, these files are available from ftp.uni-koeln.de. If you can't use ftp, see the mail server information below. Q21b: How does Fractint achieve its speed? A21b: Fractint's speed (such as it is) is due to a combination of: 1. Using fixed point math rather than floating point where possible (huge improvement for non-coprocessor machine, small for 486's). 2. Exploiting symmetry of the fractal. 3. Detecting nearly repeating orbits, avoid useless iteration (e.g. repeatedly iterating 0^2+0 etc. etc.). 4. Reducing computation by guessing solid areas (especially the "lake" area). 5. Using hand-coded assembler in many places. 6. Obtaining both sin and cos from one 387 math coprocessor instruction. 7. Using good direct memory graphics writing in 256-color modes. The first four are probably the most important. Some of these introduce errors, usually quite acceptable. ------------------------------ Subject: Fractal software Q22: Where can I obtain software packages to generate fractals? A22: For X windows: xmntns and xlmntn: these generate fractal mountains. They can be obtained from ftp: ftp.uu.net:/usenet/comp.sources.x/volume8/xmntns [137.39.1.9]. xfroot: generates a fractal root window. xmartin: generates a Martin hopalong root window. xmandel: generates Mandelbrot/Julia sets. xfroot, xmartin, xmandel are part of the X11 distribution. lyap: generates Lyapunov exponent images. Ftp from: ftp.uu.net:/usenet/comp.sources.x/volume17/lyapunov-xlib . spider: Uses Thurston's algorithm for computing postcritically finite polynomials, draws Mandelbrot and Julia sets using the Koebe algorithm, and draws Julia set external angles. Ftp from: lyapunov.ucsd.edu:pub/inls-ucsd/spider . xfractal: fractal drawing program. Ftp from: clio.rz.uni- duesseldorf.de:/X11/uploads [134.99.128.3]. Distributed X systems: MandelSpawn: computes Mandelbrot/Julia sets on a network of machines. Ftp from: export.lcs.mit.edu:/contrib [18.24.0.12] or funic.funet.fi:/pub/X11/contrib [128.214.6.100] in mandelspawn- 0.06.tar.Z. gnumandel: computes Mandelbrot images on a network. Ftp from: informatik.tu-muenchen.de:/pub/GNU/gnumandel [131.159.0.110]. For SunView: Mandtool: A Mandelbrot computing program. Ftp from: spanky.triumf.ca:/fractals/programs/mandtool ; code is in M_TAR.Z . For Unix/C: lsys: generates L-systems as PostScript or other textual output. No graphical interface at present. (in C++) Ftp from: ftp.cs.unc.edu:/pub/leech/lsys.tar.Z . lyapunov: generates PGM Lyapunov exponent images. Ftp from: ftp.uu.net:/usenet/comp.sources.misc/volume23/lyapuov . SPD: contains generators for fractal mountain, tree, recursive tetrahedron. Ftp from: princeton.edu:/pub/Graphics [128.112.128.1]. Fractal Studio: Mandelbrot set program; handles distributed computing. Ftp from archive.cs.umbc.edu:/pub/peter/fractal-studio [130.85.100.53]. Xmountains: An X11-based fractal landscape generator. Ftp from ftp.epcc.ed.ac.uk:/pub/personal/spb/xmountains . For Mac: LSystem, 3D-L-System, IFS, FracHill, Mandella and a bunch of others are available from uceng.uc.edu:/pub/wuarchive/edu/math/mac/fractals [129.137.189.1] or wuarchive.wustl.edu:/edu/math/mac/fractals . (These are also available in New Zealand at ccu1.auckland.ac.nz.) fractal-wizard.hqx, julias-dream-107.hqx, mandella-87.hqx, and others are under app in the info-mac archive: sumex-aim.stanford.edu:/info-mac [36.44.0.6], or a mirror such as plaza.aarnet.edu.au:/micros/mac/info-mac [139.130.4.6]. mandel-tv: a very fast Mandelbrot generator. Under sci at info-mac. There are also commercial programs, such as IFS Explorer and Fractal Clip Art, which are published by Koyn Software (314) 878-9125. For NeXT: Lyapunov: generates Lyapunov exponent images. Ftp from: nova.cc.purdue.edu:/pub/next/2.0-release/source . For MSDOS: DEEPZOOM: a high-precision Mandelbrot program for displaying highly zoomed fractals. Ftp from spanky.triumf.ca [142.90.112.1] in [pub.fractals.programs.ibmpc] depzm13.zip. Fractal WitchCraft: a very fast fractal design program. Ftp from: garbo.uwasa.fi:/pc/demo/fw1-08.zip [128.214.87.1]. CAL: generates more than 15 types of fractals including Mandelbrot, Lyapunov, IFS, user-defined formulas, logistic equation, and quaternion julia sets. Ftp from: oak.oakland.edu:/pub/msdos/graphics [141.210.10.117] (or any other Simtel mirror) in frcal035.zip. Fractal Discovery Laboratory: designed for use in a science museum or school setting. The Lab has five sections: Art Gallery ( 72 images -- Mandelbrots, Julias, Lyapunovs), Microscope ( 85 images -- Biomorph, Mandelbrot, Lyapunov, ...), Movies (165 images, 6 "movies": Mandelbrot Evolution, Splitting a Mini-Mandelbrot, Fractal UFO, ...), Tools (Gingerbreadman, Lorentz Equations, Fractal Ferns, von Koch Snowflake, Sierpinski Gasket), and Library (Dictionary, Books and Articles). Sampler available from Compuserver GRAPHDEV Lib 4 in DISCOV.ZIP, or send high-density disk and self-addressed, stamped envelope to: Earl F. Glynn, 10808 West 105th Street, Overland Park, Kansas 66214-3057. WL-Plot: plots functions including bifurcations and recursive relations. Ftp from wuarchive.wustl.edu:/edu/math/msdos/graphing in wlplt231.zip. There are many fractal programs available from oak.oakland.edu:/pub/msdos/graphics [141.210.10.117]: forb01a.zip: Displays orbits of Mandelbrot mapping. C/E/VGA fract30.arc: Mandelbrot/Julia set 2D/3D EGA/VGA Fractal Gen fractfly.zip: Create Fractal flythroughs with FRACTINT fdesi313.zip: Program to visually design IFS fractals frain182.zip: FRACTINT v18.1 EGA/VGA/XGA fractal generator frasr182.zip: C & ASM src for FRACTINT v18.1 fractal gen. frcal040.zip: Fractal drawing program: 15 formulae available frcaldmo.zip: 800x600x256 demo images for FRCAL030.ZIP For Windows: dy-syst.zip. This program explores Newton's method, Mandelbrot set, and Julia sets. Ftp from mathcs.emory.edu:/pub/riddle . For Amiga: (all entries marked "ff###" are .lzh files in the Fish Disk set available at ux1.cso.uiuc.edu:/amiga/fish and other sites) General Mandelbrot generators with many features: Mandelbrot (ff030), Mandel (ff218), Mandelbrot (ff239), TurboMandel (ff302), MandelBltiz (ff387), SMan (ff447), MandelMountains (ff383, in 3-D), MandelPAUG (ff452, MandFXP movies), MandAnim (ff461, anims), ApfelKiste (ff566, very fast), MandelSquare (ff588, anims) Mandelbrot and Julia sets generators: MandelVroom (ff215), Fractals (ff371, also Newton-R and other sets) With different algorithmic approaches (shown): FastGro (ff188, DLA), IceFrac (ff303, DLA), DEM (ff303, DEM), CPM (ff303, CPM in 3-D), FractalLab (ff391, any equation) Iterated Function System generators (make ferns, etc): FracGen (ff188, uses "seeds"), FCS (ff465), IFSgen (ff554), IFSLab (ff696, "Collage Theorem") Unique fractal types: Cloud (ff216, cloud surfaces), Fractal (ff052, terrain), IMandelVroom (strange attractor contours?), Landscape (ff554, scenery), Scenery (ff155, scenery), Plasma (ff573, plasma clouds) Fractal generators: PolyFractals (ff015), FFEX (ff549) Lyapunov fractals: Ftp from: ftp.luth.se:/pub/aminet/new/lyapunovia.lha [130.240.18.2]. Commercial packages: Fractal Pro 5.0, Scenery Animator 2.0, Vista Professional, Fractuality (reviewed in April '93 Amiga User International). MathVISION 2.4. Generates Julia, Mandelbrot, and others. Includes software for image processing, complex arithmetic, data display, general equation evaluation. Available for $223 from Seven Seas Software, Box 1451, Port Townsend WA 98368. Software for computing fractal dimension: Fractal Dimension Calculator is a Macintosh program which uses the box- counting method to compute the fractal dimension of planar graphical objects. Ftp from: wuarchive.wustl.edu:/edu/math/mac/fractals/FDC.sea.hqx or wuarchive.wustl.edu:/packages/architec/Fractals/FDC.sea.hqx . FD3: estimates capacity, information, and correlation dimension from a list of points. It computes log cell sizes, counts, log counts, log of Shannon statistics based on counts, log of correlations based on counts, two-point estimates of the dimensions at all scales examined, and over-all least-square estimates of the dimensions. Ftp from: lyapunov.ucsd.edu:/pub/cal-state-stan [132.239.86.10]. Also look in lyapunov.ucsd.edu:/pub/inls-ucsd for an enhanced Grassberger-Procaccia algorithm for correlation dimension. A MS-DOS version of FP3 is available by request to gentry@altair.csustan.edu. ------------------------------ Subject: Ftp questions Q23a: How does anonymous ftp work? A23a: Anonymous ftp is a method of making files available to anyone on the Internet. In brief, if you are on a system with ftp (e.g. Unix), you type "ftp lyapunov.ucsd.edu", or whatever system you wish to access. You are prompted for your name and you reply "anonymous". You are prompted for your password and you reply with your email address. You then use "ls" to list the files, "cd" to change directories, "get" to get files, and "quit" to exit. For example, you could say "cd /pub", "ls", "get README", and "quit"; this would get you the file "README". See the man page ftp(1) or ask someone at your site for more information. In this FAQ file, anonymous ftp addresses are given in the form name.of.machine:/pub/path [1.2.3.4]. The first part "name.of.machine" is the machine you must ftp to. If your machine cannot determine the host from the name, you can try the numeric Internet address: "ftp 1.2.3.4". The part after the colon: "/pub/path" is the file or directory to access once you are connected to the remote machine. Q23b: What if I can't use ftp to access files? A23b: If you don't have access to ftp because you are on a uucp/Fidonet/etc network there is an e-mail gateway at ftpmail@decwrl.dec.com that can retrieve the files for you. To get instructions on how to use the ftp gateway send a message to ftpmail@decwrl.dec.com with one line containing the word 'help'. ------------------------------ Subject: Archived pictures Q24a: Where are fractal pictures archived? A24a: Fractal images (GIFs, etc.) used to be posted to alt.fractals.pictures; this newsgroup has been replaced by alt.binaries.pictures.fractals. Pictures from 1990 and 1991 are available via anonymous ftp: csus.edu:/pub/alt.fractals.pictures [130.86.90.1]. Many Mandelbrot set images are available via anonymous ftp: ftp.ira.uka.de:/pub/graphics/fractals [129.13.10.93]. Fractal images including some recent alt.binaries.pictures.fractals images are archived at spanky.triumf.ca:/fractals [142.90.112.1]. This can also be accessed via WWW at http://spanky.triumf.ca . Some fractal images are available on the WWW at http://www.cnam.fr/fractals.html . These images are available by ftp: ftp.cnam.fr:/pub/Fractals . Fractal animations in MPG and FLI format are in ftp.cnam.fr:/pub/Fractals/anim or http://www.cnam.fr/fractals/anim.html . Another collection of fractal images is archived at ftp.maths.tcd.ie/pub/images/Computer [134.226.81.10]. Some fractal and other computer-generated images are available on the WWW at gopher://olt.et.tudelft.nl:1251/11/computer . A collection of interesting smoke- and flame-like jpeg iterated function system images is available on the WWW at http://www.cs.cmu.edu:8001/afs/cs.cmu.edu/user/spot/web/images.html . Some images are also available by ftp: hopeless.mess.cs.cmu.edu:/usr/spot/pub/film . An algorithmic image gallery is available on the WWW at http://axpba1.ba.infn.it:8080/ . Q24b: How do I view fractal pictures from alt.binaries.pictures.fractals? A24b: A detailed explanation is given in the "alt.binaries.pictures FAQ" (see "pictures-faq"). This is posted to the pictures newsgroups and is available by ftp: rtfm.mit.edu:/pub/usenet/news.answers/pictures-faq [18.181.0.24]. In brief, there is a series of things you have to do before viewing these posted images. It will depend a little on the system your working with, but there is much in common. Some newsreaders have features to automatically extract and decode images ready to display ("e" in trn) but if you don't you can use the following manual method: 1. Save/append all posted parts sequentially to one file. 2. Edit this file and delete all text segments except what is between the BEGIN-CUT and END-CUT portions. This means that BEGIN-CUT and END-CUT lines will disappear as well. There will be a section to remove for each file segment as well as the final END-CUT line. What is left in the file after editing will be bizarre garbage starting with begin 660 imagename.GIF and then about 6000 lines all starting with the letter "M" followed by a final "end" line. This is called a uuencoded file. 3. You must uudecode the uuencoded file. There should be an appropriate utility at your site; "uudecode filename" should work under Unix. Ask a system person or knowledgeable programming type. It will decode the file and produce another file called imagename.GIF. This is the image file. 4. You must use another utility to view these GIF images. It must be capable of displaying color graphic images in GIF format. (If you get a JPG format file, you may have to convert it to a GIF file with yet another utility.) In the XWindows environment, you may be able to use "xv", "xview", or "xloadimage" to view GIF files. If you aren't using X, then you'll either have to find a comparable utility for your system or transfer your file to some other system. You can use a file transfer utility such as Kermit to transfer the binary file to an IBM-PC. ------------------------------ Subject: Where can I obtain fractal papers? Q25: Where can I obtain fractal papers? A25: There are several Internet sites with fractal papers: There is an ftp archive site for preprints and programs on nonlinear dynamics and related subjects at: lyapunov.ucsd.edu:/pub [132.239.86.10]. There are also articles on dynamics, including the IMS preprint series, available from math.sunysb.edu:/preprints [129.49.31.57]. A collection of short papers on fractal formulas, drawing methods, and transforms is available by ftp: ftp.coe.montana.edu:/pub/fractals (this site hasn't been working lately). The site life.anu.edu.au [150.203.38.74] has a collection of fractal programs, papers, information related to complex systems, and gopher and World Wide Web connections. The ftp path is life.anu.edu.au:/pub/complex_systems ; look in fractals, tutorial, and anu92. The Word Wide Web access is "http://life.anu.edu.au/complex_systems/complex.html". The gopher path is: Name=BioInformatics gopher at ANU Host=life.anu.edu.au Type=1 Port=70 Path=1/complex_systems/fractals The WWW site http://legendre.ucsd.edu/Research/Fisher/complex.html has some fractal papers; they are also available by ftp: legendre.ucsd.edu:/pub/Research/Fisher . One WWW site listing many other sites related to complex systems is http://www.seas.upenn.edu/~ale/cplxsys.html . ------------------------------ Subject: How can I join the BITNET fractal discussion? Q26: How can I join the BITNET fractal discussion? A26: There is a fractal discussion on BITNET that uses an automatic mail server that sends mail to a distribution list. (On some systems, the contents of FRAC-L appear in the Usenet newsgroup bit.listserv.frac-l.) Note that once you join, you may have a very difficult time unsubscribing. To join the mailing list, send a message to listserv@gitvm1.gatech.edu with the following as text: SUBSCRIBE FRAC-L John Doe (where John Doe is replaced by your name) To unsubscribe, send the message: UNSUBSCRIBE FRAC-L If that doesn't unsubscribe you, you can try: SIGNOFF FRAC-L (GLOBAL If that doesn't work or you have other problems, you can contact the list administrator. You can obtain their name by sending the message: REVIEW FRAC-L ------------------------------ Subject: References Q27: What are some general references on fractals and chaos? A27: Some references are: 1. M. Barnsley, _Fractals Everywhere_, Academic Press Inc., 1988. ISBN 0- 12-079062-9. This is an excellent text book on fractals. This is probably the best book for learning about the math underpinning fractals. It is also a good source for new fractal types. 2. M. Barnsley and L. Anson, _The Fractal Transform_, Jones and Bartlett, April, 1993. ISBN 0-86720-218-1. This book is a sequel to _Fractals Everywhere_. Without assuming a great deal of technical knowledge, the authors explain the workings of the Fractal Transform (tm). The Fractal Transform is the compression tool for storing high-quality images in a minimal amount of space on a computer. Barnsley uses examples and algorithms to explain how to transform a stored pixel image into its fractal representation. 3. R. Devaney and L. Keen, eds., _Chaos and Fractals: The Mathematics Behind the Computer Graphics_, American Mathematical Society, Providence, RI, 1989. This book contains detailed mathematical descriptions of chaos, the Mandelbrot set, etc. 4. R. L. Devaney, _An Introduction to Chaotic Dynamical Systems_, Addison- Wesley, 1989. ISBN 0-201-13046-7. This book introduces many of the basic concepts of modern dynamical systems theory and leads the reader to the point of current research in several areas. It goes into great detail on the exact structure of the logistic equation and other 1-D maps. The book is fairly mathematical using calculus and topology. 5. R. L. Devaney, _Chaos, Fractals, and Dynamics_, Addison-Wesley, 1990. ISBN 0-201-23288-X. This is a very readable book. It introduces chaos fractals and dynamics using a combination of hands-on computer experimentation and precalculus math. Numerous full-color and black and white images convey the beauty of these mathematical ideas. 6. R. Devaney, _A First Course in Chaotic Dynamical Systems, Theory and Experiment_, Addison Wesley, 1992. A nice undergraduate introduction to chaos and fractals. 7. G. A. Edgar, _Measure Topology and Fractal Geometry_, Springer- Verlag Inc., 1990. ISBN 0-387-97272-2. This book provides the math necessary for the study of fractal geometry. It includes the background material on metric topology and measure theory and also covers topological and fractal dimension, including the Hausdorff dimension. 8. K. Falconer, _Fractal Geometry: Mathematical Foundations and Applications_, Wiley, New York, 1990. 9. J. Feder, _Fractals_, Plenum Press, New York, 1988. This book is recommended as an introduction. It introduces fractals from geometrical ideas, covers a wide variety of topics, and covers things such as time series and R/S analysis that aren't usually considered. 10. J. Gleick, _Chaos: Making a New Science_, Penguin, New York, 1987. 11. B. Hao, ed., _Chaos_, World Scientific, Singapore, 1984. This is an excellent collection of papers on chaos containing some of the most significant reports on chaos such as ``Deterministic Nonperiodic Flow'' by E.N.Lorenz. 12. S. Levy, _Artificial life : the quest for a new creation_, Pantheon Books, New York, 1992. This book takes off where Gleick left off. It looks at many of the same people and what they are doing post-Gleick. 13. B. Mandelbrot, _The Fractal Geometry of Nature_, W. H. FreeMan and Co., New York. ISBN 0-7167-1186-9. In this book Mandelbrot attempts to show that reality is fractal-like. He also has pictures of many different fractals. 14. H. O. Peitgen and P. H. Richter, _The Beauty of Fractals_, Springer- Verlag Inc., New York, 1986. ISBN 0-387-15851-0. This book has lots of nice pictures. There is also an appendix giving the coordinates and constants for the color plates and many of the other pictures. 15. H. Peitgen and D. Saupe, eds., _The Science of Fractal Images_, Springer-Verlag Inc., New York, 1988. ISBN 0-387-96608-0. This book contains many color and black and white photographs, high level math, and several pseudocoded algorithms. 16. H. Peitgen, H. Juergens and D. Saupe, _Fractals for the Classroom_, Springer-Verlag, New York, 1992. These two volumes are aimed at advanced secondary school students (but are appropriate for others too), have lots of examples, explain the math well, and give BASIC programs. 17. H. Peitgen, H. Juergens and D. Saupe, _Chaos and Fractals: New Frontiers of Science_, Springer-Verlag, New York, 1992. 18. C. Pickover, _Computers, Pattern, Chaos, and Beauty: Graphics from an Unseen World_, St. Martin's Press, New York, 1990. This book contains a bunch of interesting explorations of different fractals. 19. J. Pritchard, _The Chaos Cookbook: A Practical Programming Guide_, Butterworth-Heinemann, Oxford, 1992. ISBN 0-7506-0304-6. It contains type- in-and-go listings in BASIC and Pascal. It also eases you into some of the mathematics of fractals and chaos in the context of graphical experimentation. So it's more than just a type-and-see-pictures book, but rather a lab tutorial, especially good for those with a weak or rusty (or even non- existent) calculus background. 20. P. Prusinkiewicz and A. Lindenmayer, _The Algorithmic Beauty of Plants_, Springer-Verlag, NY, 1990. ISBN 0-387-97297-8. A very good book on L-systems, which can be used to model plants in a very realistic fashion. The book contains many pictures. 21. M. Schroeder, _Fractals, Chaos, and Power Laws: Minutes from an Infinite Paradise_, W. H. Freeman, New York, 1991. This book contains a clearly written explanation of fractal geometry with lots of puns and word play. 22. J. Sprott, _Strange Attractors: Creating Patterns in Chaos_, M&T Books (subsidary of Henry Holt and Co.), New York. " ISBN 1-55851-298-5. This book describes a new method for generating beautiful fractal patterns by iterating simple maps and ordinary differential equations. It contains over 350 examples of such patterns, each producing a corresponding piece of fractal music. It also describes methods for visualizing objects in three and higher dimensions and explains how to produce 3-D stereoscopic images using the included red/blue glasses. The accompanying 3.5" IBM-PC disk contain source code in BASIC, C, C++, Visual BASIC for Windows, and QuickBASIC for Macintosh as well as a ready-to-run IBM-PC executable version of the program. Available for $39.95 + $3.00 shipping from M&T Books (1-800-628-9658). 23. D. Stein, ed., _Proceedings of the Santa Fe Institute's Complex Systems Summer School_, Addison-Wesley, Redwood City, CA, 1988. See especially the first article by David Campbell: ``Introduction to nonlinear phenomena''. 24. R. Stevens, _Fractal Programming in C_, M&T Publishing, 1989 ISBN 1- 55851-038-9. This is a good book for a beginner who wants to write a fractal program. Half the book is on fractal curves like the Hilbert curve and the von Koch snow flake. The other half covers the Mandelbrot, Julia, Newton, and IFS fractals. 25. I. Stewart, _Does God Play Dice?: the Mathematics of Chaos_, B. Blackwell, New York, 1989. 26. T. Wegner and M. Peterson, _Fractal Creations_, The Waite Group, 1991. This is the book describing the Fractint program. Journals: "Chaos and Graphics" section in the quarterly journal _Computers and Graphics_. This contains recent work in fractals from the graphics perspective, and usually contains several exciting new ideas. "Mathematical Recreations" section by A. K. Dewdney in _Scientific American_. Algorithm - The Personal Computer Newsletter. P.O. Box 29237, Westmount Postal Outlet, 785 Wonderland Road S., London, Ontario, Canada, N6K 1M6. Fractal Report. Reeves Telecommunication Labs. West Towan House, Porthtowan, TRURO, Cornwall TR4 8AX, U.K. FRAC'Cetera. This is a gazetteer of the world of fractals and related areas, supplied in IBM PC format HD disk. For more information, contact: Jon Horner, Editor, FRAC'Cetera, Le Mont Ardaine, Rue des Ardains, St. Peters, Guernsey GY7 9EU, Channel Islands, United Kingdom. Fractals, An interdisciplinary Journal On The Complex Geometry of Nature. This is a new journal published by World Scientific. B.B Mandelbrot is the Honorary Editor and T. Vicsek, M.F. Shlesinger, M.M Matsushita are the Managing Editors). The aim of this first international journal on fractals is to bring together the most recent developments in the research of fractals so that a fruitful interaction of the various approaches and scientific views on the complex spatial and temporal behavior could take place. ------------------------------ Subject: Acknowledgements For their help with this file, thanks go to: Alex Antunes, Steve Bondeson, Erik Boman, Jacques Carette, John Corbit, Abhijit Deshmukh, Tony Dixon, Robert Drake, Detlev Droege, Gerald Edgar, Gordon Erlebacher, Yuval Fisher, Duncan Foster, David Fowler, Murray Frank, Jean-loup Gailly, Noel Giffin, Earl Glynn, Lamont Granquist, Luis Hernandez- Ure:a, Jay Hill, Arto Hoikkala, Carl Hommel, Robert Hood, Oleg Ivanov, Simon Juden, J. Kai-Mikael, Leon Katz, Matt Kennel, Tal Kubo, Jon Leech, Brian Meloon, Tom Menten, Guy Metcalfe, Eugene Miya, Lori Moore, Robert Munafo, Miriam Nadel, Ron Nelson, Tom Parker, Dale Parson, Matt Perry, Cliff Pickover, Francois Pitt, Kevin Ring, Michael Rolenz, Tom Scavo, Jeffrey Shallit, Rollo Silver, Gerolf Starke, Bruce Stewart, Dwight Stolte, Tommy Vaske, Tim Wegner, Andrea Whitlock, Erick Wong, Wayne Young, and others. Special thanks to Matthew J. Bernhardt (mjb@acsu.buffalo.edu) for collecting many of the chaos definitions. Copyright 1993,1994 Ken Shirriff (shirriff@cs.Berkeley.EDU).