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 A019290 #35 Jul 08 2025 07:31:15 %S A019290 57120,932064,3932040,251650560 %N A019290 Let sigma_m (n) be result of applying sum-of-divisors function m times to n; call n (m,k)-perfect if sigma_m (n) = k*n; sequence gives the (2,13)-perfect numbers. %C A019290 See also the Cohen-te Riele links under A019276. %C A019290 No other terms < 5*10^11. - _Jud McCranie_, Feb 08 2012 %C A019290 11383810648416 is also a term. See comment in A019278. - _Michel Marcus_, May 15 2016 %C A019290 a(5) > 4*10^12. - _Giovanni Resta_, Feb 26 2020 %C A019290 50248050278400, 117245450649600, 86575337046016000 are also terms. - _Michel Marcus_, Feb 27 2020 %H A019290 Graeme L. Cohen and Herman J. J. te Riele, <a href="http://projecteuclid.org/euclid.em/1047565640">Iterating the sum-of-divisors function</a>, Experimental Mathematics, 5 (1996), pp. 93-100. %o A019290 (PARI) isok(n) = sigma(sigma(n))/n == 13; \\ _Michel Marcus_, May 15 2016 %Y A019290 Cf. A019276, A019278, A019279, A019281, A019282, A019283, A019284, A019285, A019286, A019287, A019288, A019289, A019291. %K A019290 nonn,more %O A019290 1,1 %A A019290 _N. J. A. Sloane_