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

Terms

a(0) =0a(1) =0a(2) =2a(3) =12a(4) =48a(5) =160a(6) =480a(7) =1344a(8) =3584a(9) =9216a(10) =23040a(11) =56320a(12) =135168a(13) =319488a(14) =745472a(15) =1720320a(16) =3932160a(17) =8912896a(18) =20054016a(19) =44826624a(20) =99614720a(21) =220200960a(22) =484442112a(23) =1061158912a(24) =2315255808a(25) =5033164800a(26) =10905190400a(27) =23555211264a(28) =50734301184a(29) =108984795136