[Ipopt] Too few degrees of freedom

[Stefan Vigerske]

Hi,

> I am using the MUMPS solver with IpOpt.  My log file shows the following message:
> 
> Too few degrees of freedom (n_x = 0, n_c = 1).
>   Trying fixed_variable_treatment = RELAX_BOUNDS
> 
> What exactly does this mean?

Probably the problem that you have put into Ipopt has all variables 
fixed? Internally, Ipopt substitutes all fixed variables, so that in 
your case it ends up with n_x = 0 variables.
But since there seem to be n_c = 1 constraints, it does not see "enough 
degrees of freedom" to satisfy this constraint.
fixed_variable_treatment = RELAX_BOUNDS then means probably that Ipopt 
internally relaxes the bounds on the fixed variables a bit so that they 
can vary in a small interval.

Please note that I'm not sure about this, it's just my guess. :-)

Best,
Stefan

-- 
Stefan Vigerske
Humboldt University Berlin, Numerical Mathematics
http://www.math.hu-berlin.de/~stefan