Hi there!
I've a big Laplacian matrix for which I need to find the smallest k eigenvectors, there's a way to do it with Eigen?

Thank you very much!

There is nothing implemented in Eigen to do this. One possibility is to do power iterations. There are examples there:

viewtopic.php?f=74&t=108033#p251878

and there:

viewtopic.php?f=74&t=117176&p=290601&hilit=power+iteration#p290620

Another possibility is to use ARPACK (or another external library). There is an ArpackSupport module in unsupported for sparse symmetric eigenvalue problem which may or may not be useful.