a(n) = (8*(2^n) - n^2 - 3*n - 6)/2.

Terms

a(0) =1a(1) =3a(2) =8a(3) =20a(4) =47a(5) =105a(6) =226a(7) =474a(8) =977a(9) =1991a(10) =4028a(11) =8112a(12) =16291a(13) =32661a(14) =65414a(15) =130934a(16) =261989a(17) =524115a(18) =1048384a(19) =2096940a(20) =4194071a(21) =8388353a(22) =16776938a(23) =33554130a(24) =67108537a(25) =134217375a(26) =268435076a(27) =536870504a(28) =1073741387a(29) =2147483181