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+128 0projecteuler/128/euler128.pl
projecteuler/128/euler128.pl
+A hexagonal tile with number 1 is surrounded by a ring of six hexagonal tiles, starting at "12 o'clock" and numbering the tiles 2 to 7 in an anticlockwise direction.
+New rings are added in the same fashion, with the next rings being numbered 8 to 19, 20 to 37, 38 to 61, and so on. The diagram below shows the first three rings.
+By finding the difference between tile n and each its six neighbours we shall define PD(n) to be the number of those differences which are prime.
+For example, working clockwise around tile 8 the differences are 12, 29, 11, 6, 1, and 13. So PD(8) = 3.
+If all of the tiles for which PD(n) = 3 are listed in ascending order to form a sequence, the 10th tile would be 271.