a(n) = 2^n*(n - 3) + 4.

Terms

a(0) =0a(1) =0a(2) =4a(3) =20a(4) =68a(5) =196a(6) =516a(7) =1284a(8) =3076a(9) =7172a(10) =16388a(11) =36868a(12) =81924a(13) =180228a(14) =393220a(15) =851972a(16) =1835012a(17) =3932164a(18) =8388612a(19) =17825796a(20) =37748740a(21) =79691780a(22) =167772164a(23) =352321540a(24) =738197508a(25) =1543503876a(26) =3221225476a(27) =6710886404a(28) =13958643716a(29) =28991029252