a(n) = floor(exp(17/23) * n!).

Terms

a(0) =2a(1) =4a(2) =12a(3) =50a(4) =251a(5) =1507a(6) =10554a(7) =84434a(8) =759911a(9) =7599119a(10) =83590319a(11) =1003083838a(12) =13040089904a(13) =182561258657