a(n) = (7^n+1)/2.

Terms

a(0) =1a(1) =4a(2) =25a(3) =172a(4) =1201a(5) =8404a(6) =58825a(7) =411772a(8) =2882401a(9) =20176804a(10) =141237625a(11) =988663372a(12) =6920643601a(13) =48444505204a(14) =339111536425