Proposed by Toby Walsh
Combinatorial mathematics Games and puzzles
A magic hexagon consists of the numbers 1 to 19 arranged in a hexagonal pattern:
A,B,C D,E,F,G H,I,J,K,L M,N,O,P Q,R,S
We have a constraint that all diagonals sum to 38. That is,
A+B+C = D+E+F+G = ... = Q+R+S = 38, A+D+H = B+E+I+M = ... = L+P+S = 38, C+G+L = B+F+K+P = ... = H+M+Q = 38.
The problem can be generalized to other sizes. This is the diameter 5 problem.