With this option you can ask for eigenvalues (and eigenvectors).
The eigenvectors are determined for the principal
unknowns in the system (like velocity, temperature, etc.).
This option is only available after the matrices have been set up with **control_options_solver**.
And it is assumed that the matrices used in the eigenmodes determination
are __symmetric__.

With *type_of_eigen_analysis* you can determine which eigen problem will be solved.
If *type_of_eigen_analysis* is set to **-matrix**,
the eigen modes of the matrix will be solved.
Other options for *type_of_eigen_analysis* are presently not available.
(To be more clear: presently you cannot solve generalized eigenvalue problems
like vibration in mechanics, acoustics, etc.)

The eigenvalues are put in the **control_eigen_values** record.
The eigenvectors are put in the **node_eigen** records.

__Warning__: the eigensolvers we use are not tuned for finite element analysis.
So the eigensolvers may be very slow, and large amounts of memory may be consumed.

The eigenvalues and eigenvectors are deleted whenever a new timestep is
performed with **control_timesteps**. This is done since in general after a
new timestep the matrices will have changed, and so the eigenvalues are not
valid anymore.

See also **control_eigen_scale**.