a(n) = n^4 - 1.

Terms

a(0) =0a(1) =15a(2) =80a(3) =255a(4) =624a(5) =1295a(6) =2400a(7) =4095a(8) =6560a(9) =9999a(10) =14640a(11) =20735a(12) =28560a(13) =38415a(14) =50624a(15) =65535a(16) =83520a(17) =104975a(18) =130320a(19) =159999a(20) =194480a(21) =234255a(22) =279840a(23) =331775a(24) =390624a(25) =456975a(26) =531440a(27) =614655a(28) =707280a(29) =809999