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 A275374 #19 Aug 15 2021 02:21:17 %S A275374 1,2,3,7,12,15,31,56,63,127,135,168,234,240,255,260,384,504,511,720, %T A275374 819,896,992,1023,1080,1344,1512,1638,2047,2240,2352,3276,3564,3584, %U A275374 3744,3840,4095,4320,4655,7280,7448,8191,9360,10304,12825,12896,13104,14256,14725,15795,16256 %N A275374 Numbers k that divide sigma(k*(k+1)/2). %e A275374 7 is a term because 7 divides sigma(28) = 56. %t A275374 Select[Range@ 17000, Divisible[DivisorSigma[1, # (# + 1)/2], #] &] (* _Michael De Vlieger_, Jul 25 2016 *) %o A275374 (PARI) isok(n) = sigma(n*(n+1)/2) % n == 0 %Y A275374 Cf. A000203, A000217, A074285. %K A275374 nonn %O A275374 1,2 %A A275374 _Altug Alkan_, Jul 25 2016