(2*7^n - 3*3^n + 1)/6.

Terms

a(0) =0a(1) =1a(2) =12a(3) =101a(4) =760a(5) =5481a(6) =38852a(7) =273421a(8) =1918320a(9) =13441361a(10) =94128892a(11) =659020341a(12) =4613496680a(13) =32295539641a(14) =226071966132