(11*9^n+1)/4.

Terms

a(0) =3a(1) =25a(2) =223a(3) =2005a(4) =18043a(5) =162385a(6) =1461463a(7) =13153165a(8) =118378483a(9) =1065406345a(10) =9588657103a(11) =86297913925a(12) =776681225323