[Cbc] Artificial variables

[Christos]

Thank you very much for you time and your valuable help.

Yes, all the variables are bounded.. Some of them have equal low and 
upper bounds. I tried by differing them, but the problem remains...

I would try the option of limiting the objective but the code is too 
complex to add a new constrain in a few hours.. (The model is 
constructed in C++ by reading an excel file).

So, i almost agree that the defect is infeasibility.

Can you suggest any trick to find it while trying to limit the objective?

Thank you very much.


On 10/8/2012 20:06, acw at ascent.com wrote:
> The objective is a linear combination of some set of variables.  Of 
> course I don't know what your objective function is, but let's call it F.
>
> Since this is a minimization problem, I propose adding one more 
> constraint to the problem, F >= -1e6.  If unboundedness is the 
> problem, the modified problem will solve with no trouble.
>
> In your first response, you mention variable bounds.  Do you have 
> bounds on all your variables?  If you do, then my analysis is probably 
> wrong and the difficulty is infeasibility rather than unboundedness.
>
> It should not be necessary to know any solver internals to figure out 
> what's wrong.
>
>
> From: 	Christos <chtsolak at gmail.com>
> To: 	acw at ascent.com
> Cc: 	"cbc at list.coin-or.org" <cbc at list.coin-or.org>, Allan Wechsler 
> <acw at robson.ascent.com>
> Date: 	08/10/2012 12:37 PM
> Subject: 	Re: [Cbc] Artificial variables
>
>
> ------------------------------------------------------------------------
>
>
>
> I forgot to mention that it is a minimization problem , so i do not 
> know if there is any point in limiting objective.
>
> Sorry for the double mail.
>
> On 10/8/2012 19:24, _acw at ascent.com_ <mailto:acw at ascent.com>wrote:
> No single constraint is responsible for an infeasibility.  Consider a 
> system with one variable, and two constraints, X >= 7 and X <= 4. 
>  This system is obviously infeasible, but which of the two constraints 
> is at fault?
>
> In your case I suspect that the problem is unbounded rather than 
> infeasible, because you have so few constraints and so many variables. 
>  Unless the problem has a very special structure, a problem with fewer 
> constraints than variables is likely to be unbounded.
>
> You can check this by adding a constraint that limits your objective 
> to, say, 1e6.  If the problem now solves, then unboundedness is almost 
> certainly your difficulty.
>
> From:	Christos _<chtsolak at gmail.com>_ <mailto:chtsolak at gmail.com>
> To:	_cbc at list.coin-or.org_ <mailto:cbc at list.coin-or.org>
> Date:	08/10/2012 09:32 AM
> Subject:	[Cbc] Artificial variables
>
>
>
> ------------------------------------------------------------------------
>
>
>
> Hi, i have a model with around 10000 variable and 1500 equations.
>
> But i get the messages on the photo when i run it:_
> __http://imageshack.us/photo/my-images/39/dascd.png/_
>
> Is there any way to use artificial variable or something else in order
> to find the problematic equation?
>
> Thank you in advance
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>

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