a(n) is the 10^n-th semiprime.

Terms

a(0) =4a(1) =26a(2) =314a(3) =3595a(4) =40882a(5) =459577a(6) =5109839a(7) =56168169a(8) =611720495a(9) =6609454805a(10) =70937808071a(11) =757060825018