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A267894 Numbers whose number of odd divisors is nonprime. 2

%I #21 Dec 03 2020 06:58:19

%S 1,2,4,8,15,16,21,27,30,32,33,35,39,42,45,51,54,55,57,60,63,64,65,66,

%T 69,70,75,77,78,84,85,87,90,91,93,95,99,102,105,108,110,111,114,115,

%U 117,119,120,123,125,126,128,129,130,132,133,135,138,140,141,143,145,147,150

%N Numbers whose number of odd divisors is nonprime.

%H Amiram Eldar, <a href="/A267894/b267894.txt">Table of n, a(n) for n = 1..10000</a>

%e The divisors of 42 are 1, 2, 3, 6, 7, 14, 21, 42. The odd divisors of 42 are 1, 3, 7, 21. There are 4 odd divisors of 42 and 4 is a nonprime number, so 42 is in the sequence.

%t Select[Range[150], !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 Cf. A000079, A001227, A018252, A028982, A028983, A131651, A230577, A266531, A267891, A267892, A267893, A267895.

%K nonn

%O 1,2

%A _Omar E. Pol_, Apr 04 2016

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Last modified March 28 09:04 EDT 2024. Contains 371240 sequences. (Running on oeis4.)