A267894 Numbers whose number of odd divisors is nonprime.

1, 2, 4, 8, 15, 16, 21, 27, 30, 32, 33, 35, 39, 42, 45, 51, 54, 55, 57, 60, 63, 64, 65, 66, 69, 70, 75, 77, 78, 84, 85, 87, 90, 91, 93, 95, 99, 102, 105, 108, 110, 111, 114, 115, 117, 119, 120, 123, 125, 126, 128, 129, 130, 132, 133, 135, 138, 140, 141, 143, 145, 147, 150

Offset

1,2

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Example

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.

Mathematica

Select[Range[150], !PrimeQ[DivisorSigma[0, #/2^IntegerExponent[#, 2]]] &] (* Amiram Eldar, Dec 03 2020 *)

Prog

(PARI) isok(n) = ! isprime(sumdiv(n, d, (d%2))); \\ Michel Marcus, Apr 04 2016

Crossrefs

Cf. A000079, A001227, A018252, A028982, A028983, A131651, A230577, A266531, A267891, A267892, A267893, A267895.

Sequence in context: A097100 A002954 A019278 * A084345 A084561 A277166

Adjacent sequences: A267891 A267892 A267893 * A267895 A267896 A267897

Keyword

nonn

Author

Omar E. Pol, Apr 04 2016

Status

approved