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 A072869 #28 Dec 21 2024 00:43:46
%S A072869 1,1,4,1,12,1,8,7,15,1,31,1,18,13,24,1,32,1,36,12,24,1,91,12,31,14,56,
%T A072869 1,96,1,32,24,42,14,72,1,36,18,93,1,120,1,90,48,42,1,140,15,44,32,72,
%U A072869 1,144,18,127,24,63,1,280,1,54,42,104,20,168,1,90,40,114,1,168,1,90,57
%N A072869 a(n) = sigma(sigma(n)-n), where sigma = A000203, sum of the divisors of n.
%H A072869 Antti Karttunen, <a href="/A072869/b072869.txt">Table of n, a(n) for n = 2..16384</a>
%H A072869 <a href="/index/Su#sums_of_divisors">Index entries for sequences related to sums of divisors</a>
%F A072869 a(n) = A000203(A001065(n)). - _Michel Marcus_, Nov 26 2013
%F A072869 For p prime, a(p) = 1. - _Michel Marcus_, Nov 19 2014
%F A072869 a(k) = k if k is one greater than a Mersenne prime. - _Jarrod G. Sage_, Dec 07 2024
%t A072869 Table[DivisorSigma[1,DivisorSigma[1,n]-n],{n,2,80}] (* _Harvey P. Dale_, Sep 23 2013 *)
%o A072869 (PARI) A072869(n) = sigma(sigma(n)-n); \\ _Antti Karttunen_, Oct 30 2017
%Y A072869 Cf. A000203 (sigma(n)), A001065 (sigma(n) - n), A051027.
%Y A072869 Cf. A072868 (fixed points).
%K A072869 easy,nonn
%O A072869 2,3
%A A072869 _Benoit Cloitre_, Jul 27 2002