%I #6 Jan 14 2017 01:52:16
%S 1,6,19,44,89,158,263,408,606,867,1208,1629,2162,2807,3584,4510,5615,
%T 6893,8394,10107,12076,14321,16874,19723,22938,26527,30517,34930,
%U 39831,45188,51109,57563,64616,72301,80642,89657,99462,110027,121404,133613
%N Number of 2 X 2 nonsingular 0..n matrices with a(1,1) <= a(1,2) <= a(2,1) <= a(2,2).
%C Singular matrices = binomial(n+4,4) - a(n).
%e All solutions for n=2:
%e ..0..1....0..2....1..1....0..1....1..2....0..1
%e ..1..1....2..2....1..2....2..2....2..2....1..2
%K nonn
%O 1,2
%A _R. H. Hardin_, Jan 06 2011