a(n) = 3*trinomial(n+1,0) - trinomial(n+2,0).

Terms

a(0) =0a(1) =2a(2) =2a(3) =6a(4) =12a(5) =30a(6) =72a(7) =182a(8) =464a(9) =1206a(10) =3170a(11) =8426a(12) =22596a(13) =61074a(14) =166194a(15) =454950a(16) =1251984a(17) =3461574a(18) =9611190a(19) =26787378a(20) =74916660a(21) =210178458a(22) =591347988a(23) =1668172842a(24) =4717282752a(25) =13369522250a(26) =37970114702a(27) =108045430902