a(n) = 2^(n+1) - (n-1)^2.

Terms

a(0) =1a(1) =4a(2) =7a(3) =12a(4) =23a(5) =48a(6) =103a(7) =220a(8) =463a(9) =960a(10) =1967a(11) =3996a(12) =8071a(13) =16240a(14) =32599a(15) =65340a(16) =130847a(17) =261888a(18) =523999a(19) =1048252a(20) =2096791a(21) =4193904a(22) =8388167a(23) =16776732a(24) =33553903a(25) =67108288a(26) =134217103a(27) =268434780a(28) =536870183a(29) =1073741040