a(n) = (4*n^2 - 1)^3.

Terms

a(0) =27a(1) =3375a(2) =42875a(3) =250047a(4) =970299a(5) =2924207a(6) =7414875a(7) =16581375a(8) =33698267a(9) =63521199a(10) =112678587a(11) =190109375a(12) =307546875a(13) =480048687a(14) =726572699a(15) =1070599167a(16) =1540798875a(17) =2171747375a(18) =3004685307a(19) =4088324799a(20) =5479701947a(21) =7245075375