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

Terms

a(0) =1a(1) =41a(2) =457a(3) =1345a(4) =4481a(5) =8521a(6) =18985a(7) =30017a(8) =54721a(9) =78121a(10) =126281a(11) =168961a(12) =252097a(13) =322505a(14) =454441a(15) =562561a(16) =759425a(17) =916777a(18) =1197001a(19) =1416641a(20) =1800961a(21) =2097481a(22) =2608937a(23) =2998465a(24) =3662401a(25) =4162601a(26) =5006665a(27) =5636737a(28) =6690881a(29) =7471561