1/8 the number of 2 X 2 -n..n arrays with a 2 X 2 -n..n inverse, i.e., with determinant +-1.

Terms

a(0) =5a(1) =13a(2) =29a(3) =45a(4) =77a(5) =93a(6) =141a(7) =173a(8) =221a(9) =253a(10) =333a(11) =365a(12) =461a(13) =509a(14) =573a(15) =637a(16) =765a(17) =813a(18) =957a(19) =1021a(20) =1117a(21) =1197a(22) =1373a(23) =1437a(24) =1597a(25) =1693a(26) =1837a(27) =1933a(28) =2157a(29) =2221