a(n) = (3n+2)^7.

Terms

a(0) =128a(1) =78125a(2) =2097152a(3) =19487171a(4) =105413504a(5) =410338673a(6) =1280000000a(7) =3404825447a(8) =8031810176a(9) =17249876309a(10) =34359738368a(11) =64339296875a(12) =114415582592a(13) =194754273881a(14) =319277809664a(15) =506623120463a(16) =781250000000