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 A115909 #17 Apr 06 2023 06:30:18 %S A115909 1,2,4,8,16,32,57,64,65,85,87,128,256,512,1024,1245,1421,2048,3146, %T A115909 3330,3790,4096,6695,7257,8192,10137,16384,25884,32768,34420,34551, %U A115909 34947,65536,131072,208495,262144,348161,440495,524288,530270,534430 %N A115909 Numbers k such that sigma(k)*k is a triangular number. %C A115909 Infinite, since all the powers of 2 belong to the sequence. %H A115909 Amiram Eldar, <a href="/A115909/b115909.txt">Table of n, a(n) for n = 1..100</a> %e A115909 sigma(1421)*1421 = 2429910 = T(2204). %t A115909 Select[Range[540000],IntegerQ[(Sqrt[1+8(#*DivisorSigma[1,#])]-1)/2]&] (* _Harvey P. Dale_, Dec 16 2011 *) %o A115909 (PARI) isok(n) = ispolygonal(n*sigma(n), 3); \\ _Amiram Eldar_, Apr 06 2023 %Y A115909 Cf. A064987, A000217, A115904, A115910. %K A115909 nonn %O A115909 1,2 %A A115909 _Giovanni Resta_, Feb 06 2006