a(n) = floor(exp(1/21) * n!).

Terms

a(0) =1a(1) =2a(2) =6a(3) =25a(4) =125a(5) =755a(6) =5285a(7) =42286a(8) =380578a(9) =3805780a(10) =41863584a(11) =502363009a(12) =6530719126a(13) =91430067771