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

Terms

a(0) =1a(1) =4a(2) =12a(3) =34a(4) =96a(5) =274a(6) =792a(7) =2314a(8) =6816a(9) =20194a(10) =60072a(11) =179194a(12) =535536a(13) =1602514a(14) =4799352a(15) =14381674a(16) =43112256a(17) =129271234a(18) =387682632a(19) =1162785754a(20) =3487832976a(21) =10462450354a(22) =31385253912a(23) =94151567434a(24) =282446313696a(25) =847322163874