a(n) = 13*3^n.

Terms

a(0) =13a(1) =39a(2) =117a(3) =351a(4) =1053a(5) =3159a(6) =9477a(7) =28431a(8) =85293a(9) =255879a(10) =767637a(11) =2302911a(12) =6908733a(13) =20726199a(14) =62178597a(15) =186535791a(16) =559607373a(17) =1678822119a(18) =5036466357a(19) =15109399071a(20) =45328197213a(21) =135984591639a(22) =407953774917