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

Terms

a(0) =3a(1) =6a(2) =14a(3) =30a(4) =60a(5) =116a(6) =222a(7) =426a(8) =824a(9) =1608a(10) =3162a(11) =6254a(12) =12420a(13) =24732a(14) =49334a(15) =98514a(16) =196848a(17) =393488a(18) =786738a(19) =1573206a(20) =3146108a(21) =6291876a(22) =12583374a(23) =25166330a(24) =50332200a(25) =100663896a(26) =201327242a(27) =402653886a(28) =805307124a(29) =1610613548