Monsky's theorem
In geometry, Monsky's theorem states that it is not possible to dissect a square into an odd number of triangles of equal area.[1] In other words, a square does not have an odd equidissection.
The problem was posed by Fred Richman in the American Mathematical Monthly in 1965 and was proved by Paul Monsky in 1970.[2][3][4]
Proof
Monsky's proof combines combinatorial and algebraic techniques and in outline is as follows:
- Take the square to be the unit square with vertices at (0, 0), (0, 1), (1, 0) and (1, 1). If there is a dissection into n triangles of equal area, then the area of each triangle is 1/n.
- Colour each point in the square with one of three colours, depending on the 2-adic valuation of its coordinates.
- Show that a straight line can contain points of only two colours.
- Use Sperner's lemma to show that every triangulation of the square into triangles meeting edge-to-edge must contain at least one triangle whose vertices have three different colours.
- Conclude from the straight-line property that a tricolored triangle must also exist in every dissection of the square into triangles, not necessarily meeting edge-to-edge.
- Use Cartesian geometry to show that the 2-adic valuation of the area of a triangle whose vertices have three different colours is greater than 1. So every dissection of the square into triangles must contain at least one triangle whose area has a 2-adic valuation greater than 1.
- If n is odd, then the 2-adic valuation of 1/n is 1, so it is impossible to dissect the square into triangles all of which have area 1/n.[5]
Optimal dissections
By Monsky's theorem, it is necessary to have triangles with different areas to dissect a square into an odd number of triangles. Lower bounds for the area differences that must occur to dissect a square into an odd numbers of triangles and the optimal dissections have been studied.[6][7][8]
Generalizations
The theorem can be generalized to higher dimensions: an n-dimensional hypercube can only be divided into simplices of equal volume if the number of simplices is a multiple of n!.[2]
References
- ↑ Aigner, Martin; Ziegler, Günter M. (2010). "One square and an odd number of triangles". Proofs from The Book (4th ed.). Berlin: Springer-Verlag. pp. 131–138. doi:10.1007/978-3-642-00856-6_20. ISBN 978-3-642-00855-9.
- ↑ 2.0 2.1 Template:Cite tech report
- ↑ Monsky, P. (1970). "On Dividing a Square into Triangles". The American Mathematical Monthly 77 (2): 161–164. doi:10.2307/2317329.
- ↑ Stein, S. (2004). Kleber, M.; Vakil, R.. eds. "Cutting a Polygon into Triangles of Equal Areas". The Mathematical Intelligencer 26: 17–21. doi:10.1007/BF02985395.
- ↑ Verrill, H. A. (September 8, 2004). "Dissecting a square into triangles". Louisiana State University. http://www.math.lsu.edu/~verrill/teaching/math7280/triangles.pdf.
- ↑ Mansow, K. (2003), Ungerade Triangulierungen eines Quadrats von kleiner Diskrepanz (en. Odd triangulations of a square of small discrepancy), Germany: TU Berlin.
- ↑ Schulze, Bernd (1 July 2011). "On the area discrepancy of triangulations of squares and trapezoids". Electronic Journal of Combinatorics 18 (1): #P137. doi:10.37236/624. http://www.combinatorics.org/ojs/index.php/eljc/article/view/v18i1p137.
- ↑ Labbé, Jean-Philippe; Rote, Günter; Ziegler, Günter M. (2018). "Area Difference Bounds for Dissections of a Square into an Odd Number of Triangles". Experimental Mathematics 29 (3): 1–23. doi:10.1080/10586458.2018.1459961.
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