View source for Wallace–Bolyai–Gerwien theorem

{{short description|Theorem on polygon dissections}}
[[File:Triangledissection.svg|thumb|right|By the Wallace–Bolyai–Gerwien theorem, a [[square]] can be cut into parts and rearranged into a [[triangle]] of equal area.]]

In [[geometry]], the '''Wallace–Bolyai–Gerwien theorem''',<ref>{{Cite journal |last=Gardner |first=R. J. |date=1985-02-01 |title=A problem of Sallee on equidecomposable convex bodies |url=http://www.ams.org/jourcgi/jour-getitem?pii=S0002-9939-1985-0784187-9 |journal=Proceedings of the American Mathematical Society |language=en |volume=94 |issue=2 |pages=329–332 |doi=10.1090/S0002-9939-1985-0784187-9 |issn=0002-9939 |jstor=2045399|doi-access=free }}</ref> named after [[William Wallace (mathematician)|William Wallace]], [[Farkas Bolyai]] and [[P. Gerwien]], is a theorem related to [[Dissection problem|dissections]] of [[polygon]]s. It answers the question when one polygon can be formed from another by cutting it into a finite number of pieces and recomposing these by [[Translation (geometry)|translations]] and [[rotation]]s. The Wallace–Bolyai–Gerwien theorem states that this can be done [[if and only if]] two polygons have the same [[area]].

[[William Wallace (mathematician)|Wallace]] had proven the same result already in 1807.

According to other sources, Bolyai and Gerwien had independently proved the theorem in 1833 and 1835, respectively.

== Formulation ==
There are several ways in which this theorem may be formulated. The most common version uses the concept of "equidecomposability" of polygons: two polygons are equidecomposable if they can be split into [[finite set|finitely many]] [[triangle]]s that only differ by some [[isometry]] (in fact only by a combination of a translation and a rotation). In this case the Wallace–Bolyai–Gerwien theorem states that two polygons are equidecomposable if and only if they have the same area.

Another formulation is in terms of [[scissors congruence]]: two polygons are scissors-congruent if they can be decomposed into finitely many polygons that are pairwise [[congruence (geometry)|congruent]]. Scissors-congruence is an [[equivalence relation]]. In this case the Wallace–Bolyai–Gerwien theorem states that the [[equivalence classes]] of this relation contain precisely those polygons that have the same area.

== Proof sketch ==
The theorem can be understood in a few steps. Firstly, every polygon can be cut into triangles. There are a few methods for this. For [[convex polygon]]s one can cut off each [[vertex (geometry)|vertex]] in turn, while for [[concave polygon]]s this requires more care. A general approach that works for non-simple polygons as well would be to choose a [[line (geometry)|line]] not parallel to any of the sides of the polygon and draw a line parallel to this one through each of the vertices of the polygon. This will divide the polygon into triangles and [[trapezoids]], which in turn can be converted into triangles.

Secondly, each of these triangles can be transformed into a right triangle and subsequently into a [[rectangle]] with one side of length 1. Alternatively, a triangle can be transformed into one such rectangle by first turning it into a [[parallelogram]] and then turning this into such a rectangle. By doing this for each triangle, the polygon can be decomposed into a rectangle with unit width and height equal to its area.

Since this can be done for any two polygons, a "common subdivision" of the rectangle in between proves the theorem. That is, cutting the common rectangle (of size 1 by its area) according to both polygons will be an intermediate between both polygons.

== Notes about the proof ==
First of all, this proof requires an intermediate polygon. In the formulation of the theorem using scissors-congruence, the use of this intermediate can be reformulated by using the fact that scissor-congruences are transitive. Since both the first polygon and the second polygon are scissors-congruent to the intermediate, they are scissors-congruent to one another.

The proof of this theorem is constructive and doesn't require the [[axiom of choice]], even though some other dissection problems (e.g. [[Tarski's circle-squaring problem]]) do need it. In this case, the decomposition and reassembly can actually be carried out "physically": the pieces can, in theory, be [[Jordan curve|cut with scissors]] from paper and reassembled by hand.

Nonetheless, the number of pieces required to compose one polygon from another using this procedure generally far exceeds the minimum number of polygons needed.<ref>{{Cite web|url=http://mathworld.wolfram.com/Dissection.html|title = Dissection}}</ref>

== Degree of decomposability ==
Consider two equidecomposable polygons ''P'' and ''Q''. The minimum number ''n'' of pieces required to compose one polygon ''Q'' from another polygon ''P'' is denoted by σ(''P'',''Q'').

Depending on the polygons, it is possible to estimate upper and lower bounds for σ(''P'',''Q''). For instance, [[Alfred Tarski]] proved that if ''P'' is convex and the [[Diameter of a set|diameters]] of ''P'' and ''Q'' are respectively given by d(''P'') and d(''Q''), then<ref name=":0">{{Cite book|title=Alfred Tarski|last1=McFarland|first1=Andrew|last2=McFarland|first2=Joanna|last3=Smith|first3=James T.|date=2014|publisher=Birkhäuser, New York, NY|isbn=9781493914739|pages=77–91|language=en|doi=10.1007/978-1-4939-1474-6_5}}</ref>
:<math>\sigma(P,Q) \ge \frac{d(P)}{d(Q)}.</math>
If ''P<sub>x</sub>'' is a rectangle of sides ''a''{{thin space}}·{{thin space}}''x'' and ''a''{{thin space}}·{{thin space}}(1/''x'') and ''Q'' is a square of side length ''a'', then ''P<sub>x</sub>'' and ''Q'' are equidecomposable for every ''x'' > 0. An upper bound for σ(''P<sub>x</sub>'',''Q'') is given by<ref name=":0" />
:<math>\sigma(P_x,Q) \le 2 + \left\lceil \sqrt{x^2 - 1} \right\rceil, \quad\text{for } x \ge 1.</math>
Since σ(''P<sub>x</sub>'',''Q'') = σ(''P''<sub>(1/''x'')</sub>,''Q''), we also have that
:<math>\sigma\left(P_\frac{1}{x},Q\right) \le 2 + \left\lceil \frac{\sqrt{1-x^2}}{x} \right\rceil, \quad\text{for } x \le 1.</math>

== Generalisations ==
The analogous statement about [[polyhedron|polyhedra]] in three dimensions, known as [[Hilbert's third problem]], is false, as proven by [[Max Dehn]] in 1900. The problem has also been considered in some [[Non-Euclidean geometry|non-Euclidean geometries]]. In two-dimensional [[hyperbolic geometry|hyperbolic]] and [[spherical geometry|spherical]] geometry, the theorem holds. However, the problem is still open for these geometries in three dimensions.

==References==
{{reflist}}

==External links==
* [http://www.cut-the-knot.org/do_you_know/Bolyai.shtml Wallace–Bolyai–Gerwien Theorem]
* [https://dmsm.github.io/scissors-congruence/ Scissors Congruence] - An interactive demonstration of the Wallace–Bolyai–Gerwien theorem.
* [https://www.youtube.com/watch?v=ysV6iF3Rmjo Video showing a sketch of the proof]
* [http://demonstrations.wolfram.com/AnExampleOfTheBolyaiGerwienTheorem/ An Example of the Bolyai–Gerwien Theorem] by Sándor Kabai, Ferenc Holló Szabó, and [[Lajos Szilassi]], the [[Wolfram Demonstrations Project]].
* [http://www.math.csi.cuny.edu/abhijit/623/scissors_slides.pdf A presentation about Hilbert's third problem at College of Staten Island CUNY] -  Abhijit Champanerkar.
*  [http://demonstrations.wolfram.com/TarskisAdaptationOfWojtowiczsArgumentOnOptimalDissectionOfAU/ Optimal dissection of a unit square in a rectangle]

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[[Category:Euclidean plane geometry]]
[[Category:Theorems in discrete geometry]]
[[Category:Geometric dissection]]