Index Of Triangle 2009 ✯ | TRUSTED |
Proof sketch: (A^3)_{ii} counts walks of length 3 starting and ending at i; in simple graphs each triangle contributes 6 such walks; summing diagonal and dividing by 6 yields t. Suppose a contest defines index(I) of triangle ABC as I = floor((angle A)/(π/9)) + floor((angle B)/(π/9)) + floor((angle C)/(π/9)). For any triangle angles sum π, possible I values can be enumerated and optimized; constructive arguments and bounding yield the full distribution.
