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

Terms

a(0) =1a(1) =3a(2) =8a(3) =19a(4) =42a(5) =89a(6) =184a(7) =375a(8) =758a(9) =1525a(10) =3060a(11) =6131a(12) =12274a(13) =24561a(14) =49136a(15) =98287a(16) =196590a(17) =393197a(18) =786412a(19) =1572843a(20) =3145706a(21) =6291433a(22) =12582888a(23) =25165799a(24) =50331622a(25) =100663269a(26) =201326564a(27) =402653155a(28) =805306338a(29) =1610612705