This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A259240 #10 Feb 16 2025 08:33:25
%S A259240 36,81,590,5286126,15880,1932821,37990539325,6280,234222782808,
%T A259240 3350529,931738,455621651099,3312,2680,373569353,1128231876,
%U A259240 47531850550,601657,4609261,115668,164642040082433296,336577944,40161257476,5031720,31424211,25785,12670237746
%N A259240 Least n-gonal number greater than 1 such that sigma(n) is also n-gonal.
%H A259240 Hiroaki Yamanouchi, <a href="/A259240/b259240.txt">Table of n, a(n) for n = 3..85</a>
%H A259240 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PolygonalNumber.html">Polygonal Number</a>
%e A259240 For n=4, 81 is a square and sigma(81)=121 is also a square.
%o A259240 (PARI) a(n) = {k = 2; while(! ((p = k*((n-2)*k-(n-4))/2) && ispolygonal(sigma(p), n)), k++); k;}
%Y A259240 Cf. A000203, A008848, A083674.
%K A259240 nonn
%O A259240 3,1
%A A259240 _Michel Marcus_, Jun 22 2015
%E A259240 a(23)-a(29) from _Hiroaki Yamanouchi_, Sep 26 2015