Which of the following hexagons with cells on each edge can be filled with consecutive integers starting with (not starting with ) in such a way that the numbers in each row (in all three directions) add up to a magic constant ?
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The number of hexagons in a grid of side n is 3 n ( n − 1 ) + 1 , so the sum of numbers from 0 to 3 n ( n − 1 ) is 2 3 n ( n − 1 ) ( 3 n 2 − 3 n + 1 ) , and so the magic number should be 2 ( 2 n − 1 ) 3 n ( n − 1 ) ( 3 n 2 − 3 n + 1 ) = 3 2 ( 2 n − 1 ) 3 [ ( 2 n − 1 ) 2 − 1 ] ( 3 ( 2 n − 1 ) 2 + 1 ] Since 2 n − 1 and [ ( 2 n − 1 ) 2 − 1 ] [ 3 ( 2 n − 1 ) 2 + 1 ] are coprime, we see that M is an integer only when 2 n − 1 divides 3 , so when n = 2 .
If n = 2 then since neighbouring edge hexagons sum to the magic number M = 7 , any two non-consecutive edge hexagons must be equal to each other. Thus the seven hexagons cannot be filled with seven distinct numbers, so the case n = 2 is not possible, either.
There is no value of n ≥ 2 for which a 0 -based magic hexagon is possible.