2*3^(n-1)-(-1)^n.

Terms

a(0) =3a(1) =5a(2) =19a(3) =53a(4) =163a(5) =485a(6) =1459a(7) =4373a(8) =13123a(9) =39365a(10) =118099a(11) =354293a(12) =1062883a(13) =3188645a(14) =9565939a(15) =28697813a(16) =86093443a(17) =258280325a(18) =774840979a(19) =2324522933a(20) =6973568803a(21) =20920706405a(22) =62762119219a(23) =188286357653a(24) =564859072963