E.g.f. 3x(1+x-x^2)/(1-x).

Terms

a(0) =0a(1) =3a(2) =12a(3) =18a(4) =72a(5) =360a(6) =2160a(7) =15120a(8) =120960a(9) =1088640a(10) =10886400a(11) =119750400a(12) =1437004800a(13) =18681062400a(14) =261534873600