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 A371920 #13 Apr 26 2025 20:27:40
%S A371920 24,30,42,54,60,66,78,84,90,96,102,112,114,120,126,132,138,140,150,
%T A371920 156,168,174,176,180,186,198,204,208,210,216,222,224,228,234,240,246,
%U A371920 252,258,264,270,276,280,282,294,304,306,308,312,318,330,336,342,348,354,360
%N A371920 Abundant numbers whose abundance is also an abundant number.
%C A371920 First differs from A125639 at n = 12.
%C A371920 Numbers k such that A033880(k) > 0 and A033880(A033880(k)) > 0.
%C A371920 This sequence is infinite: if m is divisible by 6 and coprime to 5, then 5*m is a term.
%C A371920 All the multiply-perfect numbers (A007691) that are not 1 or perfect (A000396), i.e., the terms of A166069, are terms of this sequence.
%H A371920 Amiram Eldar, <a href="/A371920/b371920.txt">Table of n, a(n) for n = 1..10000</a>
%e A371920 24 is a term since A033880(24) = 12 > 0 and A033880(12) = 4 > 0.
%t A371920 ab[n_] := DivisorSigma[1, n] - 2*n; q[n_] := Module[{k = ab[n]}, k > 0 && ab[k] > 0]; Select[Range[360], q]
%o A371920 (PARI) ab(n) = sigma(n) - 2*n;
%o A371920 is(n) = {my(k = ab(n)); k > 0 && ab(k) > 0;}
%Y A371920 Cf. A033880 (abundance), A000396, A007691, A125639.
%Y A371920 Subsequence of A005101.
%Y A371920 Subsequences: A141545, A166069, A223610, A223611, A223613.
%K A371920 nonn,easy
%O A371920 1,1
%A A371920 _Amiram Eldar_, Apr 12 2024