Number of zero-sum -n..n arrays of 6 elements with first and second differences also in -n..n.

Terms

a(0) =11a(1) =129a(2) =643a(3) =2191a(4) =5963a(5) =13635a(6) =27647a(7) =51753a(8) =90021a(9) =147955a(10) =233373a(11) =353731a(12) =518711a(13) =742015a(14) =1035117a(15) =1413269a(16) =1897685a(17) =2504735a(18) =3256189a(19) =4183023a(20) =5306711a(21) =6657117a(22) =8276669a(23) =10192753a(24) =12444053a(25) =15087213a(26) =18155235a(27) =21697465a(28) =25786411