a(n) = (3^(2*n+1) - 8*n - 3)/16.

Terms

a(0) =0a(1) =1a(2) =14a(3) =135a(4) =1228a(5) =11069a(6) =99642a(7) =896803a(8) =8071256a(9) =72641337a(10) =653772070a(11) =5883948671a(12) =52955538084a(13) =476599842805