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

Terms

a(0) =1a(1) =6a(2) =19a(3) =48a(4) =109a(5) =234a(6) =487a(7) =996a(8) =2017a(9) =4062a(10) =8155a(11) =16344a(12) =32725a(13) =65490a(14) =131023a(15) =262092a(16) =524233a(17) =1048518a(18) =2097091a(19) =4194240a(20) =8388541a(21) =16777146a(22) =33554359a(23) =67108788a(24) =134217649a(25) =268435374a(26) =536870827a(27) =1073741736a(28) =2147483557a(29) =4294967202