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

Terms

a(0) =1a(1) =57a(2) =377a(3) =1345a(4) =3553a(5) =7737a(6) =14937a(7) =26177a(8) =42945a(9) =66681a(10) =99193a(11) =142209a(12) =198241a(13) =269049a(14) =357593a(15) =466433a(16) =598401a(17) =756281a(18) =944057a(19) =1164289a(20) =1421601a(21) =1719161a(22) =2061081a(23) =2451329a(24) =2895489a(25) =3396729a(26) =3960569a(27) =4591937a(28) =5296289a(29) =6077881