A065523 - OEIS

%I #6 Mar 30 2012 17:34:49

%S 3,7,21,31,81,93,127,217,381,400,651,889,2667,3937,8191,11811,24573,

%T 27559,57337,82677,131071,172011,253921,393213,524287,761763,917497,

%U 1040257,1572861,1777447,2752491,3120771,3670009,4063201,5332341

%N Numbers n such that sigma(n) is a prime power (A025475).

%H Donovan Johnson, <a href="/A065523/b065523.txt">Table of n, a(n) for n = 1..80</a>

%t s[n_] := DivisorSigma[1, n ]; Select[ Range[2, 10^6], !PrimeQ[ s[ # ]] && Mod[ s[ # ], s[ # ] - EulerPhi[ s[ # ]]] == 0 & ]

%Y A proper subset of A065496.

%K nonn

%O 1,1

%A _Robert G. Wilson v_, Nov 27 2001

%E a(31)-a(35) from _Donovan Johnson_, Sep 05 2008