Number of 2 X 2 nonsingular 0..n matrices with a(1,1) <= a(1,2) <= a(2,1) <= a(2,2).

Terms

a(0) =1a(1) =6a(2) =19a(3) =44a(4) =89a(5) =158a(6) =263a(7) =408a(8) =606a(9) =867a(10) =1208a(11) =1629a(12) =2162a(13) =2807a(14) =3584a(15) =4510a(16) =5615a(17) =6893a(18) =8394a(19) =10107a(20) =12076a(21) =14321a(22) =16874a(23) =19723a(24) =22938a(25) =26527a(26) =30517a(27) =34930a(28) =39831a(29) =45188