[SCIP] Transformed Problem + Infeasible solution at Constraint Handler

[Gregor Hendel]

Hi Matheus,

your reasoning about negation is correct, so I would expect that the LP 
solution satisfies ** at CONSENFOLP. A special case occurs if the 
knapsack constraint is flagged by (not initial/not separate/not check), 
in which case the constraint is not immediately part of the root LP and 
might not be separated at violation.

Note that the knapsack constraint handler has a very low enforcement 
priority, -600000. Probably, the enforcement priority of your constraint 
handler is higher, and the knapsack constraint handler would add its 
constraint to the LP in the same enforcement round, but later?

Kind regards,
Gregor

Am 06.12.19 um 01:01 schrieb Matheus Ota:
> Hi all,
>
> First I would like to say that I'm very grateful for all the help I 
> received from this mailing list! I have a question regarding a MIP 
> model that I'm solving. I have some equalities of the following type
>
>     x_i,0 + x_i,1 == 1, for i = 0, ..., 8 (*)
>
>
> Thus, SCIP presolve routines replace some of my variables (x_i,0) by 
> the negation of other variables (1 - x_i,1). In the transformed model 
> I have only this constraint
>
>     [knapsack] <R>: +1<~t_x_8,1>[B] +1<~t_x_7,1>[B] +1<~t_x_6,1>[B]
>     +1<~t_x_5,1>[B] +1<~t_x_4,1>[B] +1<~t_x_3,1>[B] +1<~t_x_2,1>[B]
>     +1<~t_x_1,1>[B] +1<~t_x_0,1>[B] <= 4;
>
>
>  And each variable x is binary. The tilde (~) means negation here? If 
> so, replacing <~t_x_i,1> by <t_x_i,0> this would give
>
>     <t_x_8,0> + ... + <t_x_0,0><= 4 (**)
>
>
> But at the CONSENFOLP method, SCIP is entering with a "solution" S 
> with x_i,0 = 1 and x_i,1 = 0, for i = 0, ..., 8. Which is infeasible 
> because violates (**). But the SCIP documentation says that CONSENFOLP 
> is called when an LP solution is available. But this solution S do not 
> respect the constraints in my LP. I'm a little confused with this 
> behavior and would be glad if someone could give me some light here.
>
> Thanks!
> Matheus
>
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