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 A205597 #36 Jan 13 2025 04:11:01 %S A205597 1,15,21,1023,29127,550095,355744082763 %N A205597 Odd terms of A019278: odd n such that sigma(sigma(n))/n is an integer. %C A205597 a(8) > 4*10^12, if it exists. - _Giovanni Resta_, Feb 26 2020 %C A205597 First five terms are squarefree. Sigma(sigma(n))/n ratios for these seven known terms are: 1, 4, 3, 4, 4, 6, 4. - _Antti Karttunen_, Mar 19 2021 %H A205597 <a href="/index/O#opnseqs">Index entries for sequences where odd perfect numbers must occur, if they exist at all</a> %e A205597 15 is odd, sigma(15) = 24, sigma(24) = 60, and 60/15 is an integer. %t A205597 Select[Range[1, 10^6, 2], Mod[Nest[DivisorSigma[1, #] &, #, 2], #] == 0 &] (* _Michael De Vlieger_, Mar 19 2021 *) %o A205597 (PARI) isok(n) = (n%2) && (denominator(sigma(sigma(n))/n) == 1); \\ _Michel Marcus_, Sep 27 2017 %Y A205597 Cf. A000203, A019278, A019283, A228058, A323653. %K A205597 nonn,more %O A205597 1,2 %A A205597 _Jud McCranie_, Feb 08 2012