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 A325651 #17 Jul 26 2025 08:07:55
%S A325651 1,2,3,4,5,11,7,8,9,17,11,12,13,23,23,25,17,18,19,41,31,22,23,59,25,
%T A325651 41,27,39,29,71,31,32,47,53,47,36,37,59,39,89,41,77,43,83,45,71,47,75,
%U A325651 49,50,71,97,53,95,71,95,79,89,59,167,61,77,103,64,83,119
%N A325651 a(n) = greatest k such that sigma(k) = sigma(n).
%H A325651 Amiram Eldar, <a href="/A325651/b325651.txt">Table of n, a(n) for n = 1..10000</a>
%e A325651 a(6) = 11 because sigma(6) = sigma(11) = 12.
%t A325651 a[n_] := Block[{s = DivisorSigma[1, n], k}, k=s; While[ DivisorSigma[1, k] != s, k--]; k]; Array[a, 66] (* _Giovanni Resta_, May 20 2019 *)
%o A325651 (Magma) [Max([k: k in[1..10000] | SumOfDivisors(k) eq SumOfDivisors(n)]): n in [1..100]];
%o A325651 (PARI) a(n) = {my(s=sigma(n)); forstep(i=s, 1, -1, if (sigma(i) == s, return(i)););} \\ _Michel Marcus_, May 12 2019
%Y A325651 Cf. A000203, A275987 (least k such that sigma(k) = sigma(n)).
%Y A325651 See A070242, A325652 and A325653 for number, sum and product of such numbers k.
%K A325651 nonn
%O A325651 1,2
%A A325651 _Jaroslav Krizek_, May 12 2019