a(n) = (2n+3)! /( n! * (n+1)! ).

Terms

a(0) =6a(1) =60a(2) =420a(3) =2520a(4) =13860a(5) =72072a(6) =360360a(7) =1750320a(8) =8314020a(9) =38798760a(10) =178474296a(11) =811246800a(12) =3650610600a(13) =16287339600a(14) =72129646800a(15) =317370445920