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

Terms

a(0) =1a(1) =5a(2) =35a(3) =287a(4) =2501a(5) =22265a(6) =199655a(7) =1794707a(8) =16145801a(9) =145292525a(10) =1307573675a(11) =11767985927a(12) =105911341901a(13) =953200482785