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

Terms

a(0) =2a(1) =6a(2) =13a(3) =26a(4) =51a(5) =100a(6) =197a(7) =390a(8) =775a(9) =1544a(10) =3081a(11) =6154a(12) =12299a(13) =24588a(14) =49165a(15) =98318a(16) =196623a(17) =393232a(18) =786449a(19) =1572882a(20) =3145747a(21) =6291476a(22) =12582933a(23) =25165846a(24) =50331671a(25) =100663320a(26) =201326617a(27) =402653210a(28) =805306395a(29) =1610612764