a(n) = 1 + C(2*n,n) + C(3*n,n).

Terms

a(0) =3a(1) =6a(2) =22a(3) =105a(4) =566a(5) =3256a(6) =19489a(7) =119713a(8) =748342a(9) =4735446a(10) =30229772a(11) =194242153a(12) =1254381857a(13) =8132826045a(14) =52900345681a(15) =345022543105