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

Terms

a(0) =4a(1) =7a(2) =15a(3) =37a(4) =99a(5) =277a(6) =795a(7) =2317a(8) =6819a(9) =20197a(10) =60075a(11) =179197a(12) =535539a(13) =1602517a(14) =4799355a(15) =14381677a(16) =43112259a(17) =129271237a(18) =387682635a(19) =1162785757a(20) =3487832979a(21) =10462450357a(22) =31385253915a(23) =94151567437a(24) =282446313699a(25) =847322163877