a(n) is the least semiprime > n!.

Terms

a(0) =4a(1) =4a(2) =9a(3) =25a(4) =121a(5) =721a(6) =5041a(7) =40321a(8) =362885a(9) =3628801a(10) =39916803a(11) =479001617a(12) =6227020801a(13) =87178291201