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 A115904 #12 Jul 15 2021 21:24:20 %S A115904 7,10,66,136,145,196,198,234,270,438,495,528,592,606,855,910,1045, %T A115904 1158,1428,2208,2718,2922,3016,3663,3808,3916,4506,4975,5043,5136, %U A115904 5598,6366,6600,7080,7498,7821,8172,8200,8217,8814,9318,9730,9738 %N A115904 Numbers k such that k + sigma(k) is a triangular number. %H A115904 Harvey P. Dale, <a href="/A115904/b115904.txt">Table of n, a(n) for n = 1..1000</a> %e A115904 234 + sigma(234) = 780 = T(39). %t A115904 trnoQ[n_]:=IntegerQ[(Sqrt[1+8n]-1)/2]; Select[Range[10000], trnoQ[#+ DivisorSigma[ 1,#]]&] (* _Harvey P. Dale_, Apr 26 2014 *) %o A115904 (PARI) isok(n) = ispolygonal(n + sigma(n), 3); \\ _Michel Marcus_, Jan 25 2014 %Y A115904 Cf. A069066, A115907, A115909. %K A115904 nonn %O A115904 1,1 %A A115904 _Giovanni Resta_, Feb 06 2006