Hi all,

Suppose I have a matrix . I compute its eigenmodes

.

are eigenvectors and eigenvalues of size . The eigenvalues are put in a descending order.

Now I cut off several eigenmodes (the ones having small value), it becomes

.

What I want is to reconstruct from .

It seems that in Matlab, if I just modify the above equation

,

it will not work.

So my question is: how can I reliably reconstruct the original matrix by using its reduced eigenmodes? Thanks!!