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A259240

Least n-gonal number greater than 1 such that sigma(n) is also n-gonal.

36, 81, 590, 5286126, 15880, 1932821, 37990539325, 6280, 234222782808, 3350529, 931738, 455621651099, 3312, 2680, 373569353, 1128231876, 47531850550, 601657, 4609261, 115668, 164642040082433296, 336577944, 40161257476, 5031720, 31424211, 25785, 12670237746

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OFFSET

3,1

LINKS

Hiroaki Yamanouchi, Table of n, a(n) for n = 3..85

Eric Weisstein's World of Mathematics, Polygonal Number

EXAMPLE

For n=4, 81 is a square and sigma(81)=121 is also a square.

PROG

(PARI) a(n) = {k = 2; while(! ((p = k*((n-2)*k-(n-4))/2) && ispolygonal(sigma(p), n)), k++); k; }

CROSSREFS

Cf. A000203, A008848, A083674.