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

Terms

a(0) =0a(1) =0a(2) =1a(3) =0a(4) =-1a(5) =1a(6) =0a(7) =2a(8) =-4a(9) =4a(10) =-3a(11) =6a(12) =-9a(13) =9a(14) =-8a(15) =12a(16) =-16a(17) =16a(18) =-15a(19) =20a(20) =-25a(21) =25a(22) =-24a(23) =30a(24) =-36a(25) =36a(26) =-35a(27) =42a(28) =-49a(29) =49