a(n) = (7*n + 1)^6.

Terms

a(0) =1a(1) =262144a(2) =11390625a(3) =113379904a(4) =594823321a(5) =2176782336a(6) =6321363049a(7) =15625000000a(8) =34296447249a(9) =68719476736a(10) =128100283921a(11) =225199600704a(12) =377149515625a(13) =606355001344a(14) =941480149401