a(0) = 1; a(n) = (5*3^(n-1) - 1)/2 for n > 0.

Terms

a(0) =1a(1) =2a(2) =7a(3) =22a(4) =67a(5) =202a(6) =607a(7) =1822a(8) =5467a(9) =16402a(10) =49207a(11) =147622a(12) =442867a(13) =1328602a(14) =3985807a(15) =11957422a(16) =35872267a(17) =107616802a(18) =322850407a(19) =968551222a(20) =2905653667a(21) =8716961002a(22) =26150883007a(23) =78452649022a(24) =235357947067a(25) =706073841202