a(n) = number of semiprimes < 10^n.

Terms

a(0) =0a(1) =3a(2) =34a(3) =299a(4) =2625a(5) =23378a(6) =210035a(7) =1904324a(8) =17427258a(9) =160788536a(10) =1493776443a(11) =13959990342a(12) =131126017178