A225375 Odd numbers with exactly 2 distinct prime factors.

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

R. J. Mathar, Oct 13 2008

Keywords

• nonn

Comments

Numbers of the form p^i*q^j where p, q are distinct odd primes and i>=1, j>=1.

Subset of A098905 (which contains in addition A046390 and numbers like 255255, 285285, 345345, 373065 etc.).

Crossrefs

Subsequence of A272592. A046388 is a subsequence.

Cf. A046315, A046390, A046404, A024556, A056913.

Different from A046388, A098905.

Programs

Maple

isA225375 := proc(n) RETURN( (n mod 2 = 1) and (A001221(n) = 2) ); end proc:
for n from 1 to 840 do if isA225375(n) then printf("%d,",n) ; end if; end do:

Mathematica

Select[2 Range[200] + 1, PrimeNu[#] == 2&] (* Jean-François Alcover, Apr 04 2020 *)

PARI

is_A046388(n)={ bittest(n,0) & omega(n)==2 } \\ M. F. Hasler, Feb 13 2012

Extensions

Entry created by N. J. A. Sloane, May 09 2013 in order to restore the original definition of A046388.