Floor( exp(4/21)*n! ).

Terms

a(0) =1a(1) =2a(2) =7a(3) =29a(4) =145a(5) =871a(6) =6097a(7) =48780a(8) =439021a(9) =4390215a(10) =48292365a(11) =579508382a(12) =7533608975a(13) =105470525661