a(n) = floor(exp(18/23) * n!).

Terms

a(0) =2a(1) =4a(2) =13a(3) =52a(4) =262a(5) =1574a(6) =11023a(7) =88186a(8) =793680a(9) =7936804a(10) =87304847a(11) =1047658165a(12) =13619556146a(13) =190673786051