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 A320425 #32 Mar 20 2022 18:09:24 %S A320425 1,2,4,8,16,19,38,76,152,304,608,1216,1824,3744,3840,4864,6400,7904, %T A320425 11520,14592,19200,21888,23712,24320,25536,32768,33696,34560,43776, %U A320425 71136,72960,80640,102144,103680,114688,121600,134400,134784,213408,218880,306432,311296,364800,403200 %N A320425 Numbers k such that sigma(sigma(k^4)) == 0 (mod k^2). %C A320425 Inspired by _Allan C. Wechsler_ in seqfan list, Jan 07 2019. %C A320425 Are 1 and 19 the only odd terms? %H A320425 Robert G. Wilson v, <a href="/A320425/b320425.txt">Table of n, a(n) for n = 1..70</a> %t A320425 fQ[n_] := Mod[DivisorSigma[1, DivisorSigma[1, n^4]], n^2] == 0; Select[Range@ 476671, fQ] %o A320425 (PARI) isok(n) = (sigma(sigma(n^4)) % n^2) == 0; \\ _Michel Marcus_, Jan 09 2019 %Y A320425 Cf. A202994 (sigma(n^4)). %K A320425 nonn %O A320425 1,2 %A A320425 _Robert G. Wilson v_, Jan 08 2019