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 A019280 #28 Dec 27 2021 21:09:13 %S A019280 1,2,4,6,12,16,18,30,60 %N A019280 Let sigma_m(n) be result of applying the sum-of-divisors function m times to n; call n (m,k)-perfect if sigma_m(n) = k*n; sequence gives log_2 of the (2,2)-perfect numbers. %C A019280 Cohen and te Riele prove that any even (2,2)-perfect number (a "superperfect" number) must be of the form 2^(p-1) with 2^p-1 prime (Suryanarayana) and the converse also holds. Any odd superperfect number must be a perfect square (Kanold). Searches up to > 10^20 did not find any odd examples. - _Ralf Stephan_, Jan 16 2003 %C A019280 See also the Cohen-te Riele links under A019276. %H A019280 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. %F A019280 Coincides with A000043(n) - 1 unless odd superperfect numbers exist. %Y A019280 Cf. A019278, A019279. %K A019280 nonn,more %O A019280 1,2 %A A019280 _N. J. A. Sloane_ %E A019280 a(8)-a(9) from _Jud McCranie_, Jun 01 2000