In the documention of eigs:
Using eigs is not the most efficient way to find a few eigenvalues of small, dense matrices. If the problem fits into memory, it might be quicker to use eig(full(A)). For example, finding three eigenvalues in a 500-by-500 matrix is a relatively small problem that is easily handled with eig.
It seems not very obvious to me:
To find the 3 smallest eigenvalues of A. What `eigs` does is simply eigs(A,3,'sm').
However, eig does not have a command like this. I browse through the whole eig documentation page and all I know is to do eig(full(A)) and do the sorting on all the total number of eigenvalues myself and extract the 3 smallest out. Unlike the one-liner eigs(A,3,'sm'), this takes quite a number of steps for this purpose and it is only a little bit faster.
From the quote above, it seems to me that there is an undocumented very fast way to find the 3 smallest eigenvalues from eig, without finding all of the eigenvalues first.
I tried: eig(full(A),3,'sm') and eig(full(A),3). But they do not work.
