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

Terms

a(0) =2a(1) =4a(2) =9a(3) =22a(4) =57a(5) =154a(6) =429a(7) =1222a(8) =3537a(9) =10354a(10) =30549a(11) =90622a(12) =269817a(13) =805354a(14) =2407869a(15) =7207222a(16) =21588897a(17) =64701154a(18) =193972389a(19) =581655022a(20) =1744440777a(21) =5232273754a(22) =15694724109a(23) =47079978022a(24) =141231545457a(25) =423677859154