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 A256150 #14 Nov 13 2024 05:37:29
%S A256150 2,12,56,342,992,16256,17822,169332,628056,1189190,2720850,11085570,
%T A256150 35599122,67100672,1147210770,1317435912,1707135806,7800334080,
%U A256150 11208986256,13366943840,17109032402,17179738112,46343540900,58413331032,83717924940,204574837700,274877382656,445968192672,589130699852
%N A256150 Oblong numbers n such that sigma(n) is a triangular number.
%C A256150 The numbers 12, 56, 992, 16256, 67100672, ... (A139256(n), twice even perfect numbers) are in the sequence because they are oblong (A139256(n) = 2^k*(2^k-1)) and sigma(A139256(n)) = sigma(2^k*(2^k-1)) = sigma(2^k)*sigma(2^k-1) = (2^(k+1)-1)*2^(k+1)/2 is a triangular number.
%C A256150 This sequence is the intersection of A002378 and A045746.
%e A256150 2 is in the sequence because 2=1*2 is oblong, and sigma(2) = 1+2 = 3 = 2*3/2 is triangular.
%t A256150 Select[2 Accumulate[Range@10000], MemberQ[Accumulate[Range@10000], DivisorSigma[1, #]] &] (* _Michael De Vlieger_, Mar 17 2015 *)
%o A256150 (PARI) {for (i=1,i=10^6,n=i*(i+1);if(ispolygonal(sigma(n), 3),print(n)))}
%Y A256150 Cf. A000217, A002378, A139256, A045746, A256149, A256151, A256152.
%K A256150 nonn
%O A256150 1,1
%A A256150 _Antonio Roldán_, Mar 16 2015