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 A065523 #6 Mar 30 2012 17:34:49 %S A065523 3,7,21,31,81,93,127,217,381,400,651,889,2667,3937,8191,11811,24573, %T A065523 27559,57337,82677,131071,172011,253921,393213,524287,761763,917497, %U A065523 1040257,1572861,1777447,2752491,3120771,3670009,4063201,5332341 %N A065523 Numbers n such that sigma(n) is a prime power (A025475). %H A065523 Donovan Johnson, <a href="/A065523/b065523.txt">Table of n, a(n) for n = 1..80</a> %t A065523 s[n_] := DivisorSigma[1, n ]; Select[ Range[2, 10^6], !PrimeQ[ s[ # ]] && Mod[ s[ # ], s[ # ] - EulerPhi[ s[ # ]]] == 0 & ] %Y A065523 A proper subset of A065496. %K A065523 nonn %O A065523 1,1 %A A065523 _Robert G. Wilson v_, Nov 27 2001 %E A065523 a(31)-a(35) from _Donovan Johnson_, Sep 05 2008