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Numbers n such that sigma(n) is a power of prime (of the form p^a, p prime, a>=1).
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%I #12 Feb 09 2017 06:12:32

%S 2,3,4,7,9,16,21,25,31,64,81,93,127,217,289,381,400,651,729,889,1681,

%T 2401,2667,3481,3937,4096,5041,7921,8191,10201,11811,15625,17161,

%U 24573,27559,27889,28561,29929,57337,65536,82677,83521,85849

%N Numbers n such that sigma(n) is a power of prime (of the form p^a, p prime, a>=1).

%C Contains all Mersenne primes (A000668) and squarefree products thereof (A046528 except 1). - _Ivan Neretin_, Feb 08 2017

%H Ivan Neretin, <a href="/A070763/b070763.txt">Table of n, a(n) for n = 1..10000</a>

%e sigma(57337)=2^16 hence 57337 is in the sequence.

%t Select[Range[100000], PrimePowerQ[DivisorSigma[1, #]] &] (* _Ivan Neretin_, Feb 08 2017 *)

%o (PARI) for(n=1,100000,if(omega(sigma(n)) == 1,print1(n,",")))

%Y Cf. A071114.

%K easy,nonn

%O 1,1

%A _Benoit Cloitre_, May 14 2002