a(n) = n^3 + (3^n+1)/2.

Terms

a(0) =1a(1) =3a(2) =13a(3) =41a(4) =105a(5) =247a(6) =581a(7) =1437a(8) =3793a(9) =10571a(10) =30525a(11) =89905a(12) =267449a(13) =799359a(14) =2394229a(15) =7177829a(16) =21527457a(17) =64574995a(18) =193716077a(19) =581137593a(20) =1743400201a(21) =5230185863a(22) =15690540453a(23) =47071601581a(24) =141214782065a(25) =423644320347