A267895 - OEIS

%I #20 Dec 03 2020 06:58:14

%S 3,5,6,7,9,10,11,12,13,14,17,18,19,20,22,23,24,25,26,28,29,31,34,36,

%T 37,38,40,41,43,44,46,47,48,49,50,52,53,56,58,59,61,62,67,68,71,72,73,

%U 74,76,79,80,81,82,83,86,88,89,92,94,96,97,98,100,101,103,104,106,107,109

%N Numbers whose number of odd divisors is prime.

%C All odd primes are in the sequence.

%H Amiram Eldar, <a href="/A267895/b267895.txt">Table of n, a(n) for n = 1..10000</a>

%e The divisors of 36 are 1, 2, 3, 4, 6, 9, 12, 18, 36. The odd divisors of 36 are 1, 3, 9. There are 3 odd divisors of 36 and 3 is prime, so 36 is in the sequence.

%t Select[Range[100], PrimeQ[DivisorSigma[0, #/2^IntegerExponent[#, 2]]] &] (* _Amiram Eldar_, Dec 03 2020 *)

%o (PARI) isok(n) = isprime(sumdiv(n, d, (d%2))); \\ _Michel Marcus_, Apr 04 2016

%Y Complement of A267894.

%Y Cf. A000040, A001227, A028982, A028983, A038550, A065091, A072502, A266531, A267696, A267697.

%K nonn

%O 1,1

%A _Omar E. Pol_, Apr 04 2016