a(n) = (4n+3)^7.

Terms

a(0) =2187a(1) =823543a(2) =19487171a(3) =170859375a(4) =893871739a(5) =3404825447a(6) =10460353203a(7) =27512614111a(8) =64339296875a(9) =137231006679a(10) =271818611107a(11) =506623120463a(12) =897410677851