A=[2 3 -1; 1 2 4; 23 5 8; -2 0 1]
B=[-2, 2, 11; 12, 1, 2; -5, 0, 4; 1, 2, 3]
A(2,3)
A(2,:)
A(:,3)
eye(3)
zeros(2,3)
ones(2,3)
D=diag([4 3 2])