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- Path: sparky!uunet!ogicse!uwm.edu!ux1.cso.uiuc.edu!news.iastate.edu!pv343f.vincent.iastate.edu!abian
- From: abian@iastate.edu (Alexander Abian)
- Newsgroups: sci.physics
- Subject: What's a MANIFOLD
- Summary: Subconscious (and thus deeper) understanding of a DIFF. MANIFOLD
- Keywords: DIFFERENTIABLE MANIFOLDS
- Message-ID: <abian.721017708@pv343f.vincent.iastate.edu>
- Date: 6 Nov 92 02:41:48 GMT
- Article-I.D.: pv343f.abian.721017708
- Sender: news@news.iastate.edu (USENET News System)
- Organization: Iowa State University, Ames IA
- Lines: 167
-
-
-
- Messrs AUSTERN, BAEZ, CHOW, FLOWER, HALL, MOORE, PRATT, RUSIN, SMITH,
- VIDUGIRIS, WEISS, WIENER
-
- I am in my office and on my bookshelf I have the following books:
-
- Differential Manifolds by G. de Rham, 1984
- Differential Manifolds by Y. Matsushima, 1982
- Differential Forms in Algebraic Topology by R.Bott and L.W. Tu, 1982
- Complex Cobordism and Stable Homotopy Groups, by D.l Ravenel, 1986
- An Introduction to Algebraic Topology by J.Rotman 1988
- Linear Spaces and Differentiation by A. Froelicher and A. Kriegl,1988
-
- Everyone of the above books has a Definition of a Differentiable Manifold.
- I also read your contributions, views, and discussions concerning various
- approaches in connection with a definition of a Differentiable Manifold.
- So, I have been exposed (in fact rather technically) to the notion
- of a Differentiable Manifold and can recite its definition as described by
- some of you and by the abovementioned books (in fact I have known them
- for many, many years!). I know, that it is desirable to have an intrinsic
- definition of a Differentiable manifold without imbeddings, etc, etc, I
- know all of that! HOWEVER, HOWEVER, HOWEVER, HOWEVER, HOWEVER,
-
- HOWEVER, if you wake me up from a deep sleep around 4 a.m. and ask me
-
- WHAT IS A 2-DIMENSIONAL DIFFERENTIABLE MANIFOLD OF A EUCL. 3-SPACE R^3
-
- I will answer you as followws which is stored in my subconscious and there-
- fore which has touched me deeply and convincingly and which is all content
- with minimum amount of noise !
-
- ANSWER: A 2-dimensional Differentiable manifold of R^3 space is the
- set of all 3-tuples (x,y,z) which satisfy some system of
- 3-equations in 2 parameters p, q of the following form:
-
-
- x = x(p,q) with parameters p, q ranging over some pairs of
- real numbers such that x(p,q), y(p,q), z(p,q)
- (1) y = y(p,q) have part. deriv. of all orders and the Jacobian
-
- z = z(p,q) | dx/dp dx/dq | has rank 2 for p,q
- | dy/dp dy/dq | ranging over the above
- | dz/dp dz/dq | pairs.
-
-
- In the Jacobian determinant all derivatives are partial. I don't have
- a "round d" on my keybord !
-
- Here is an example of a 2-diff. manifold of R^3
-
-
- x = cos p + cos q | -sin p -sin q | where this
- | | Jacobian has
- (2) y = sin p + sin q | cos p cos q | rank 2 if
- | | sin(q-p) is
- z = (-1-2cos(u-v)^(1/2) | dz/du dz/dv | not zero
-
- (it is too late and I left the third line in the Jacobian in implicit
- form. Believe me I can partial differentiate!)
-
- eliminating p, q in the above 3 equations we obtain
-
- z = (-x^2 - y^2 + 1)^(1/2) which shows that (2) is the upper shell
-
- of the unit sphere which is a 2- diff. manifold in R^3
-
-
- Now, any 2-diff manifold in R^4 space will be the set of all 4-tuples
-
- (x,y,z,t) which satisfy some system of 4 equations in 2-parameters p,q
-
- of the following form:
-
- x = x(p,q) with parameters p,q range over pairs of real num-
- bers such that the functions x(p,q), y(p,q),
- (3) y = y(p,q) z(p,q), t(p,q) have part. deriv. of all orders
- and the Jacobian
- z = z(p,q)
- | dx/dp dx/dq | has rank 2 for p,q
- t = t(p,q) | dy/dp dy/dq | ranging over the
- | dz/dp dz/dq | above pairs
- | dt/dp dt/dq |
-
-
- It has been shown, that for suitable x(p,q), y(p,q), z(p,q), t(p,q)
- (3) may describe the Klein Bottle.
-
- Thus, both the 2-sphere and KLEIN bottle are 2-dimensional diff.
- manifolds.
-
- Hovever whereas 2-sphere can be described by 3-equations (2) with
- 2 parameters ,the Klein bottle cannot be described by 3-equations in 2
- parameters ,but (as mentioned above) , the Klein bottle can be described
- by 4 equations in 2-parameters of the form (3).
-
- So, the above is the essential difference between 2-sphere and Klein
- bottle. One has to mentione that 2-sphere can also be described by 4
- equations in 2-parameters, in fact by 10 equations in 2-parameters. But
- this does not make Klein bottle and 2-spehre diffeomorphic, sincve 2
- sphere can by described by 3 equations with 2 parameters whereas Klein
- bottle cannot be so described. Indeed, Kline bottle is not diffeomorphic
- with 2-spehere.
-
- In general, if you wake me up at 4 a.m. I will give you the following
- general definition.
-
-
- DEFINITION. An m-dimensional Differentiable Manifold of R^n (of
- course with m less than or equal to n>1) is a set of n-tuples
- (x(1), x(2), x(3), ..., x(n)) which satisfy some n equations in m
- parameters p(1), p(2), p(3), ..., p(m) of the form:
-
-
- x(1) = x(1) ( p(1), p(2), p(3), ..., p(m) )
-
- x(2) = x(2) ( p(1), p(2), p(3), ..., p(m) )
-
- (4) x(3) = x(3) ( p(1), p(2), p(3), ..., p(m) )
-
- ............................................
- ............................................
-
- x(n) = x(n) ( p(1), p(2), p(3), ..., p(m) )
-
- with parameters p(1), ..., p(m) ranging over the set of m-tuples of real
- numbers such that the functions x(1), ..., x(n) have part. deriv. of all
- orders and such that that n by m Jacobian given by (5) has rank m
- when parameters range over the m-tuples.
-
-
- | dx(1)/dp(1) dx(1)/dp(2) ... dx(1)/dp(m) |
- | |
- | dx(2)/dp(1) dx(2)/dp(2) ... dx(2)/dp(m) |
- (5) | |
- | dx(3)/dp(1) dx(3)/dp(2) ... dx(3)/dp(m) |
- | |
- | .................................. |
- | .................................. |
- | |
- | dx(n)/dp(1) dx(n)/dp(2) ... dx(n)/dp(m)|
-
-
-
- Please observe that an m-diff.manifold of R^n is described by
- n equations in m parameters such that ....
-
-
-
- I only talk about m-DIFF.MANIFOLDS IN R^n (my subconscious stores
- only this setup. But it touches me deeply).
-
- It is around 1a.m., please do not assume that I do not know all the
- other Definitions of an m-diff. manifiold - At 4.am. (in fact , even
- now at around 1 a.m) I LIKE THE ONE THAT I GAVE ABOVE.
-
- Of course, the Jacobian pops up automatically when in (4) one replaces
- the column of x(1), x(2), ..., x(n) on the left of = by the
- column of dx(1), dx(2), ..., dx(n) and performs the taking of the dif-
- ferentials and rewriting the whole thing as a matrix. I have explained
- this before.
- It is 1:15 a.m. I am exhaustd , but enjoyed typing this. However,
- there may be some typos. I am in no position of rereading.
-
-
- With best wishes and regards,
- Alexander ABIAN
-