a(n) = (3*11^n - 7^n)/2.

Terms

a(0) =1a(1) =13a(2) =157a(3) =1825a(4) =20761a(5) =233173a(6) =2598517a(7) =28818985a(8) =318655921a(9) =3516744733a(10) =38764899277a(11) =426978842545