A098905 Odd numbers whose number of distinct prime factors is positive and even.

15, 21, 33, 35, 39, 45, 51, 55, 57, 63, 65, 69, 75, 77, 85, 87, 91, 93, 95, 99, 111, 115, 117, 119, 123, 129, 133, 135, 141, 143, 145, 147, 153, 155, 159, 161, 171, 175, 177, 183, 185, 187, 189, 201, 203, 205, 207, 209, 213, 215, 217, 219, 221, 225, 235, 237

Offset: 1

Author

Shyam Sunder Gupta, Oct 14 2004

Keywords

• easy
• nonn

Examples

			21 is a member of sequence because 21 is odd and number of distinct prime factors of 21, i.e., 2 is even.
		

Links

• G. C. Greubel, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A001221.

Programs

Magma

[k:k in [3..250 by 2]|IsEven(#PrimeDivisors(k))]; // Marius A. Burtea, Jan 20 2020

Mathematica

Select[Select[Range[2,400], EvenQ[PrimeNu[#]] &], OddQ] (* G. C. Greubel, May 09 2017 *)
Select[Range[3,250,2],EvenQ[PrimeNu[#]]&] (* Harvey P. Dale, Jan 19 2020 *)

PARI

isok(n) = (n>1) && (n%2) && !(omega(n) % 2); \\ Michel Marcus, Jan 20 2020