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 A015709 #12 Oct 02 2017 10:55:05 %S A015709 6,22,33,44,69,76,82,140,190,295,354,392,423,451,531,581,650,944,1072, %T A015709 1119,1142,1150,1668,2276,2893,4043,4163,4176,4192,4257,4262,4509, %U A015709 5090,5975,7260,8660,12977,14927,15026,15753,16064,19105 %N A015709 Composite n such that phi(n) * sigma(n) is one less than a square. %H A015709 Donovan Johnson, <a href="/A015709/b015709.txt">Table of n, a(n) for n = 1..500</a> %H A015709 Richard K. Guy, <a href="http://www.jstor.org/stable/2974586">Divisors and desires</a>, Amer. Math. Monthly, 104 (1997), 359-360. %o A015709 (PARI) isok(n) = !isprime(n) && issquare(eulerphi(n)*sigma(n)+1); \\ _Michel Marcus_, Oct 02 2017 %K A015709 nonn %O A015709 1,1 %A A015709 _Robert G. Wilson v_ %E A015709 Offset corrected by _Donovan Johnson_, Apr 02 2012