a(n) = (5n)^4.

Terms

a(0) =0a(1) =625a(2) =10000a(3) =50625a(4) =160000a(5) =390625a(6) =810000a(7) =1500625a(8) =2560000a(9) =4100625a(10) =6250000a(11) =9150625a(12) =12960000a(13) =17850625a(14) =24010000a(15) =31640625a(16) =40960000a(17) =52200625a(18) =65610000a(19) =81450625a(20) =100000000a(21) =121550625