a(n) = (9*n + 3)^6.

Terms

a(0) =729a(1) =2985984a(2) =85766121a(3) =729000000a(4) =3518743761a(5) =12230590464a(6) =34296447249a(7) =82653950016a(8) =177978515625a(9) =351298031616a(10) =646990183449