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 A109131 #11 Sep 21 2019 08:25:38 %S A109131 20,24,30,42,54,66,78,84,102,104,114,138,140,174,186,222,224,246,258, %T A109131 282,308,318,354,364,366,402,426,438,464,474,476,498,532,534,582,606, %U A109131 618,642,644,650,654,678,762,786,812,822,834,868,894,906,942,945,978 %N A109131 Admirable numbers such that the subtracted divisor is a triangular number. %H A109131 Amiram Eldar, <a href="/A109131/b109131.txt">Table of n, a(n) for n = 1..10000</a> %e A109131 a(2)=24 because 1+2+3+4+8+12-6 = 24 and the subtracted divisor is triangular. %t A109131 triQ[n_] := IntegerQ @ Sqrt[8n + 1]; Select[Range[1000], MemberQ[Divisors[#], (d = (DivisorSigma[1, #] - 2#)/2)] && triQ[d] &] (* _Amiram Eldar_, Sep 21 2019 *) %Y A109131 Cf. A000217 (triangular numbers), A111592 (admirable numbers). %K A109131 easy,nonn %O A109131 1,1 %A A109131 _Jason Earls_, Aug 17 2005