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

Terms

a(0) =1a(1) =3a(2) =8a(3) =21a(4) =56a(5) =153a(6) =428a(7) =1221a(8) =3536a(9) =10353a(10) =30548a(11) =90621a(12) =269816a(13) =805353a(14) =2407868a(15) =7207221a(16) =21588896a(17) =64701153a(18) =193972388a(19) =581655021a(20) =1744440776a(21) =5232273753a(22) =15694724108a(23) =47079978021a(24) =141231545456a(25) =423677859153