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- From: alopez-o@neumann.uwaterloo.ca (Alex Lopez-Ortiz)
- Subject: sci.math FAQ: Cutting a sphere
- Summary: Part 27 of many, New version,
- Originator: alopez-o@neumann.uwaterloo.ca
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- Date: Fri, 17 Nov 1995 17:15:56 GMT
- Expires: Fri, 8 Dec 1995 09:55:55 GMT
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-
- Archive-Name: sci-math-faq/AC/cuttingSphere
- Last-modified: December 8, 1994
- Version: 6.2
-
-
-
- utting a sphere into pieces of larger volume
-
-
-
- Is it possible to cut a sphere into a finite number of pieces and
- reassemble into a solid of twice the volume?
-
- This question has many variants and it is best answered explicitly.
-
- Given two polygons of the same area, is it always possible to dissect
- one into a finite number of pieces which can be reassembled into a
- replica of the other?
-
- Dissection theory is extensive. In such questions one needs to specify
-
-
- * What is a ``piece"? (polygon? Topological disk? Borel-set?
- Lebesgue-measurable set? Arbitrary?)
-
- * How many pieces are permitted (finitely many? countably?
- uncountably?)
-
- * What motions are allowed in ``reassembling" (translations?
- rotations? orientation-reversing maps? isometries? affine maps?
- homotheties? arbitrary continuous images? etc.)
-
- * How the pieces are permitted to be glued together. The simplest
- notion is that they must be disjoint. If the pieces are polygons
- [or any piece with a nice boundary] you can permit them to be
- glued along their boundaries, ie the interiors of the pieces
- disjoint, and their union is the desired figure.
-
-
-
- Some dissection results
-
- * We are permitted to cut into finitely many polygons, to translate
- and rotate the pieces, and to glue along boundaries; then yes, any
- two equal-area polygons are equi-decomposable.
-
- This theorem was proven by Bolyai and Gerwien independently, and
- has undoubtedly been independently rediscovered many times. I
- would not be surprised if the Greeks knew this.
-
- The Hadwiger-Glur theorem implies that any two equal-area polygons
- are equi-decomposable using only translations and rotations by 180
- degrees.
-
- * Theorem [Hadwiger-Glur, 1951] Two equal-area polygons P,Q are
- equi-decomposable by translations only, iff we have equality of
- these two functions: phi_P() = phi_Q()
-
-
- Here, for each direction v (ie, each vector on the unit circle in
- the plane), let phi_P(v) be the sum of the lengths of the edges of
- P which are perpendicular to v , where for such an edge, its
- length is positive if v is an outward normal to the edge and is
- negative if v is an inward normal to the edge.
-
- * In dimension 3, the famous ``Hilbert's third problem" is:
-
- If P and Q are two polyhedra of equal volume, are they
- equi-decomposable by means of translations and rotations, by cutting
- into finitely many sub-polyhedra, and gluing along boundaries?
-
-
- The answer is no and was proven by Dehn in 1900, just a few months
- after the problem was posed. (Ueber raumgleiche polyeder,
- Goettinger Nachrichten 1900, 345-354). It was the first of
- Hilbert's problems to be solved. The proof is nontrivial but does
- not use the axiom of choice.
-
-
-
- References
-
- Hilbert's Third Problem. V.G. Boltianskii. Wiley 1978.
-
-
-
- * Using the axiom of choice on non-countable sets, you can prove
- that a solid sphere can be dissected into a finite number of
- pieces that can be reassembled to two solid spheres, each of same
- volume of the original. No more than nine pieces are needed.
-
- The minimum possible number of pieces is five. (It's quite easy to
- show that four will not suffice). There is a particular dissection
- in which one of the five pieces is the single center point of the
- original sphere, and the other four pieces A , A' , B , B' are
- such that A is congruent to A' and B is congruent to B' . [See
- Wagon's book].
-
- This construction is known as the Banach-Tarski paradox or the
- Banach-Tarski-Hausdorff paradox (Hausdorff did an early version
- of it). The ``pieces" here are non-measurable sets, and they are
- assembled disjointly (they are not glued together along a
- boundary, unlike the situation in Bolyai's thm.) An excellent book
- on Banach-Tarski is:
-
- The Banach-Tarski Paradox. Stan Wagon. Cambridge University Press,
- 985
-
-
-
- Robert M. French. The Banach-Tarski theorem. The Mathematical
- Intelligencer, 10 (1988) 21-28.
-
-
-
- The pieces are not (Lebesgue) measurable, since measure is
- preserved by rigid motion. Since the pieces are non-measurable,
- they do not have reasonable boundaries. For example, it is likely
- that each piece's topological-boundary is the entire ball.
-
- The full Banach-Tarski paradox is stronger than just doubling the
- ball. It states:
-
- * Any two bounded subsets (of 3-space) with non-empty interior, are
- equi-decomposable by translations and rotations.
-
- This is usually illustrated by observing that a pea can be cut up
- into finitely pieces and reassembled into the Earth.
-
- The easiest decomposition ``paradox" was observed first by
- Hausdorff:
-
- * The unit interval can be cut up into countably many pieces which,
- by translation only, can be reassembled into the interval of
- length 2.
-
- This result is, nowadays, trivial, and is the standard example of
- a non-measurable set, taught in a beginning graduate class on
- measure theory.
-
- * Theorem. There is a finite collection of disjoint open sets in the
- unit cube in R^3 which can be moved by isometries to a finite
- collection of disjoint open sets whose union is dense in the cube
- of size 2 in R^3.
-
-
-
-
- This result is by Foreman and Dougherty.
-
-
-
-
-
- References
-
- Boltyanskii. Equivalent and equidecomposable figures. in Topics in
- Mathematics published by D.C. HEATH AND CO., Boston.
-
-
-
- Dubins, Hirsch and ? Scissor Congruence American Mathematical Monthly.
-
-
-
-
- ``Banach and Tarski had hoped that the physical absurdity of this
- theorem would encourage mathematicians to discard AC. They were
- dismayed when the response of the math community was `Isn't AC great?
- How else could we get such counterintuitive results?' ''
-
-
-
-
- _________________________________________________________________
-
-
-
- alopez-o@barrow.uwaterloo.ca
- Tue Apr 04 17:26:57 EDT 1995
-
-