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

Terms

a(0) =1a(1) =3a(2) =11a(3) =39a(4) =131a(5) =423a(6) =1331a(7) =4119a(8) =12611a(9) =38343a(10) =116051a(11) =350199a(12) =1054691a(13) =3172263a(14) =9533171a(15) =28632279a(16) =85962371a(17) =258018183a(18) =774316691a(19) =2323474359a(20) =6971471651a(21) =20916512103a(22) =62753730611a(23) =188269580439a(24) =564825518531