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 A015721 #12 Oct 02 2017 10:55:02 %S A015721 6,22,44,76,82,140,190,354,392,650,944,1072,1142,1150,1668,2276,4176, %T A015721 4192,4262,5090,7260,8660,15026,16064,22926,29746,39436,42872,74666, %U A015721 87498,94786,101882,115772,118928,127580,151348 %N A015721 Composite and even n such that phi(n) * sigma(n) is one less than a square. %H A015721 Donovan Johnson, <a href="/A015721/b015721.txt">Table of n, a(n) for n = 1..300</a> %H A015721 Richard K. Guy, <a href="http://www.jstor.org/stable/2974586">Divisors and desires</a>, Amer. Math. Monthly, 104 (1997), 359-360. %o A015721 (PARI) isok(n) = !isprime(n) && !(n%2) && issquare(eulerphi(n)*sigma(n)+1); \\ _Michel Marcus_, Oct 02 2017 %Y A015721 Subsequence of even terms of A015709. %K A015721 nonn %O A015721 1,1 %A A015721 _Robert G. Wilson v_ %E A015721 Offset corrected by _Donovan Johnson_, Jan 16 2012