a(n) = 2^(n+1) - 3*n.

Terms

a(0) =1a(1) =2a(2) =7a(3) =20a(4) =49a(5) =110a(6) =235a(7) =488a(8) =997a(9) =2018a(10) =4063a(11) =8156a(12) =16345a(13) =32726a(14) =65491a(15) =131024a(16) =262093a(17) =524234a(18) =1048519a(19) =2097092a(20) =4194241a(21) =8388542a(22) =16777147a(23) =33554360a(24) =67108789a(25) =134217650a(26) =268435375a(27) =536870828a(28) =1073741737a(29) =2147483558