Why would chol fail?

I have a positive definite sparse array. When I do the chol like

L = chol(M,'lower')
M2 = (L)*(L.')

I find that M2 is not equal to M. I mean that all the elements are equal except 4. These elements are zero in M and -.7 on M2, and vis versa. In other words, M2 has two element pairs that have switched values. I thought M2 should equal M.

I need to solve

Ax = b
Ax = c
Ax = d
...
etc.

How can I use chol to do this efficiently? I have to use stock MATLAB functions.

[Mohamed Selim]

chol function works only with Positive Definate and Symmetric square matrices.

Make sure that your matrix M is symmetric.

You can use "isequal(M , M')"

if you get "1" => M is Symmetric

If you get "0" => M is not Symmetric.