[Ipopt] Eliminated fixed variables

Aydin Buluc abuluc at lbl.gov
Mon Feb 21 23:57:44 EST 2011

Hi there,

I have a NLP with about 2n variables, of which (n' < n) are fixed (x_l =
The number of constraints are m+n, of which m are linear inequality
constraints and n are nonlinear equality constraints.

My understanding is that IPOPT makes those fixed variables parameters
instead. Leaving about 2n-n' variables to the solver.

I get a slightly different result though:
"Too few degrees of freedom (n_x = 8993, n_c = 17906)"

I don't know which part of the problem (the nonlinear part only?, or just
the equality equations?) corresponds to these n_x and n_c numbers.
I only see n-n' variables for n_x where I expect n_x = 2n-n' = 8993 +17906
= 26899.
The number of constraints (n_c) don't include m either, leaving me partially

Perhaps to help me better debug this problem, one can suggest how to access
the id's of the eliminated variables?
This way I can make sure if any of the variables are incorrectly eliminated
(presumably due to my own mistake).

- Aydin
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://list.coin-or.org/pipermail/ipopt/attachments/20110221/01169169/attachment.html 

More information about the Ipopt mailing list