a(n) = (8*n+3)^6.

Terms

a(0) =729a(1) =1771561a(2) =47045881a(3) =387420489a(4) =1838265625a(5) =6321363049a(6) =17596287801a(7) =42180533641a(8) =90458382169a(9) =177978515625a(10) =326940373369a(11) =567869252041a(12) =941480149401