Triangle Tiling V: Tilings by a tile with integer sides

TL;DR

This paper investigates triangle tilings with tiles having integer sides and a 120-degree angle, establishing lower bounds on the number of tiles needed and eliminating the possibility of certain tilings.

Contribution

It extends previous work by proving that N must be at least 96 for such tilings and removes exceptions for tiles with a 120-degree angle, leading to non-existence results for specific N.

Findings

N is at least 96 for tilings with such tiles

No tilings exist for N=7, 11, 14, 19, 31, 41

Minimal unresolved N for equilateral triangles is 135

Abstract

An N-tiling of triangle ABC by triangle T is a way of writing ABC as a union of N triangles congruent to T, overlapping only at their boundaries. The triangle T is the "tile". The tile may or may not be similar to ABC. We wish to understand possible tilings by completely characterizing the triples (ABC, T, N) such that ABC can be N-tiled by T. In particular, this understanding should enable us to specify for which N there exists a tile T and a triangle ABC that is N-tiled by T; or given N, to determine which tiles and triangles can be used for N-tilings; or given ABC, to determine which tiles and N can be used to N-tile ABC. This is the fifth paper in a series of papers on this subject. The previous papers have reduced the problem the case when T has a 120 degree angle and integer side lengths. That is the problem we take up in this paper. We are still not able to completely solve the problem, but we prove that if there are any N-tilings by such tiles, then N is at least 96. Combining this results with our earlier work, we can remove the exception for a tile with a 120 degree angle, obtaining definitive non-existence results. For example, there is no 7-tiling, no 11-tiling, no 14-tiling, no 19-tiling, no 31-tiling, no 41-tiling, etc. Regarding the number N=96: There are several possible shapes of ABC, and for each shape, we exhibit the smallest N for which it is presently unknown whether there is an N-tiling. For example, for equilateral ABC, the simplest unsolved case as of May, 2012 is N=135. For each of these minimal-N examples, the tile would have to have sides (3,5,7).