Proposed by Toby Walsh

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Combinatorial mathematics
Games and puzzles
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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.