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

1,1

Links

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

Example

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

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 *)

Prog

(PARI) isok(n) = (n>1) && (n%2) && !(omega(n) % 2); \\ Michel Marcus, Jan 20 2020
(Magma) [k:k in [3..250 by 2]|IsEven(#PrimeDivisors(k))]; // Marius A. Burtea, Jan 20 2020

Crossrefs

Cf. A001221.

Sequence in context: A275384 A185307 A061346 * A225375 A329229 A146166

Adjacent sequences: A098902 A098903 A098904 * A098906 A098907 A098908

Keyword

easy,nonn

Author

Shyam Sunder Gupta, Oct 14 2004

Status

approved