Number of elements (a b, c d) in GL(2,Z) with |det| = 1, trace <= n and 0 <= a <= {b, c} <= d.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =11a(5) =18a(6) =22a(7) =30a(8) =36a(9) =48a(10) =53a(11) =67a(12) =74a(13) =87a(14) =100a(15) =120a(16) =125a(17) =145a(18) =155a(19) =179a(20) =193a(21) =215a(22) =227a(23) =255a(24) =273a(25) =303a(26) =317a(27) =339a(28) =356a(29) =404