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

Terms

a(0) =1a(1) =2a(2) =6a(3) =16a(4) =38a(5) =84a(6) =178a(7) =368a(8) =750a(9) =1516a(10) =3050a(11) =6120a(12) =12262a(13) =24548a(14) =49122a(15) =98272a(16) =196574a(17) =393180a(18) =786394a(19) =1572824a(20) =3145686a(21) =6291412a(22) =12582866a(23) =25165776a(24) =50331598a(25) =100663244a(26) =201326538a(27) =402653128a(28) =805306310a(29) =1610612676