A=[2 1 1 0; 4 3 3 1; 8 7 9 5; 6 7 9 8 ]
[L,U] = lufact(A);
L
U
L*U - A
b = [1;2;3;4]
y = L\b
x = U\y
A\b
A = [ 1e-12 1; 1 1
A = [ 1e-12 1; 1 1 ]
norm(A)
cond(A)