a(n) is the number of squarefree semiprimes <= 2^n.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =4a(5) =7a(6) =18a(7) =37a(8) =76a(9) =149a(10) =293a(11) =575a(12) =1106a(13) =2162a(14) =4161a(15) =8068a(16) =15604a(17) =30181a(18) =58449a(19) =113179a(20) =219587a(21) =425951a(22) =827393a(23) =1608250a(24) =3128647a(25) =6090677a(26) =11867571a(27) =23139485a(28) =45148817a(29) =88155104