a(n) = 21*n^2.

Terms

a(0) =0a(1) =21a(2) =84a(3) =189a(4) =336a(5) =525a(6) =756a(7) =1029a(8) =1344a(9) =1701a(10) =2100a(11) =2541a(12) =3024a(13) =3549a(14) =4116a(15) =4725a(16) =5376a(17) =6069a(18) =6804a(19) =7581a(20) =8400a(21) =9261a(22) =10164a(23) =11109a(24) =12096a(25) =13125a(26) =14196a(27) =15309a(28) =16464a(29) =17661