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simple constraint makes model infeasible

  • 25 June 2020
  • 2 replies
  • 151 views

Hi all,

I am new to AIMMS and I was wondering if someone could help me with my model.

 

I would like to maximize profit, fulfill as much demand as possible + keeping a safety stock. Besides reorder quantity is restricted by a maximum order quantity (see Sets, Parameters, Variables and Constraints attached).

 

I linearized min(Demand, E). 

E = demand fulfilled 

However, when I introduce the maximum order quantity constraints the model becomes infeasible.

 

Besides, the listing file results in the following messages:

CPLEX 12.10 constructed the following irreducible infeasibility set (IIS).
The IIS consists of 3 constraints and 4 variables.

Stock_definition(B,NL19,p-1) .. [ 9 | 31 | IIS ]

    - 1 * X(B,NL19,p-1) + 1 * Stock(B,NL19,p-1) = -120320 ; (lhs=0) ****


Stock_definition(B,NL33,p-1) .. [ 9 | 43 | IIS ]

    - 1 * X(B,NL33,p-1) + 1 * Stock(B,NL33,p-1) = -428130 ; (lhs=0) ****


OQ_B(p-1) .. [ 9 | 152 | IIS ]

    + 1 * X(B,GB13,p-1) + 1 * X(B,NL19,p-1) + 1 * X(B,NL30,p-1) + 1 * X(B,NL33,p-1) <= 500000 ; (lhs=0)


The variables in the IIS are:

    name               lower level upper
    X(B,GB13,p-1)          0     0   inf
    X(B,NL30,p-1)          0     0   inf
    Stock(B,NL19,p-1)      0     0   inf
    Stock(B,NL33,p-1)      0     0   inf

 

Can someone explain me what to do?

 

Many thanks in advance!

 

Kind regards,

 

Claire

 

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Best answer by Marcel Hunting 26 June 2020, 11:02

Hi @ClaireAdrienne. The variable E is not shown in the two Stock_definition constraints which is strange as it is part of the definition of Stock in the model.

 

It could be that you are loading a case and that the case contains information that freezes the E variable to a certain value (and that would also explain the large negative value in the right-hand-side of Stock_definition). You could try to add the following statement before the solve statement:

 

empty E;

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2 replies

Userlevel 3

Hi @ClaireAdrienne. The variable E is not shown in the two Stock_definition constraints which is strange as it is part of the definition of Stock in the model.

 

It could be that you are loading a case and that the case contains information that freezes the E variable to a certain value (and that would also explain the large negative value in the right-hand-side of Stock_definition). You could try to add the following statement before the solve statement:

 

empty E;

Hi @ClaireAdrienne 

Was Marcel’s suggestion able to help you with your model?

 

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