Number of elements of order 2 in GL(2,Z_n).

Terms

a(0) =3a(1) =13a(2) =27a(3) =31a(4) =55a(5) =57a(6) =175a(7) =109a(8) =127a(9) =133a(10) =391a(11) =183a(12) =231a(13) =447a(14) =607a(15) =307a(16) =439a(17) =381a(18) =895a(19) =811a(20) =535a(21) =553a(22) =2463a(23) =751a(24) =735a(25) =973a(26) =1623a(27) =871a(28) =1791a(29) =993