Number of 2 X 2 matrices having all terms in {-n,...,0,...,n} and 0 <= determinant <= n.

Terms

a(0) =1a(1) =53a(2) =273a(3) =737a(4) =1613a(5) =2821a(6) =4853a(7) =7125a(8) =10593a(9) =14597a(10) =19885a(11) =25309a(12) =33677a(13) =41189a(14) =51269a(15) =62565a(16) =76145a(17) =88793a(18) =106821a(19) =122581a(20) =144541a(21) =166045a(22) =189997a(23) =212877a(24) =246653a(25) =275081a(26) =308369a(27) =343281a(28) =384977a(29) =421097