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
Offset: 3
Keywords
Examples
For n=4, 81 is a square and sigma(81)=121 is also a square.
Links
- Hiroaki Yamanouchi, Table of n, a(n) for n = 3..85
- Eric Weisstein's World of Mathematics, Polygonal Number
Programs
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PARI
a(n) = {k = 2; while(! ((p = k*((n-2)*k-(n-4))/2) && ispolygonal(sigma(p), n)), k++); k;}
Extensions
a(23)-a(29) from Hiroaki Yamanouchi, Sep 26 2015