a(n) = 2^(n-2)*binomial(n+1,2).

Terms

a(0) =3a(1) =12a(2) =40a(3) =120a(4) =336a(5) =896a(6) =2304a(7) =5760a(8) =14080a(9) =33792a(10) =79872a(11) =186368a(12) =430080a(13) =983040a(14) =2228224a(15) =5013504a(16) =11206656a(17) =24903680a(18) =55050240a(19) =121110528a(20) =265289728a(21) =578813952a(22) =1258291200a(23) =2726297600a(24) =5888802816a(25) =12683575296a(26) =27246198784a(27) =58384711680a(28) =124822487040