a(n) = binomial(4*n-1,n).

Terms

a(0) =1a(1) =3a(2) =21a(3) =165a(4) =1365a(5) =11628a(6) =100947a(7) =888030a(8) =7888725a(9) =70607460a(10) =635745396a(11) =5752004349a(12) =52251400851a(13) =476260169700