Dear members

have a model in which I have to use ceil function in one of my constraints. but when I use DNLP solvers it does not result in optimal solution. When I tested my constraints I figured out that function ceil does not operate correctly. More precisely I have a positive variable pi(k) in the constraint:

Fk(k)… sum(n $(ord(n)=t), z(k,n)) - z(k,k)-ceil(pi(k))+pi(k)=g=1;

but in .lst file I have:

Fk(n0)… (1)*pi(n0) - z(n0,n0) + z(n0,n11) =G= 1 ; (LHS = 0, INFES = 1 ****)

.

.

.

Fk(n10)… (1)*pi(n10) - z(n10,n10) + z(n10,n11) =G= 1 ;

I can’t understand why infeasibility of the constraint is 1. Can anyone help me?

Maria

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## Dear Maria

The output you show is from the model generation BEFORE the model is solved. It just means that the point from which you start is infeasible.

However, you will most likely have problems with a model with Ceil. The DNLP solvers are solving the model as an NLP and is ignoring discontinuities. You have a function that jumps and the solver will not be able to predict the jumps. If you want to create a â€˜goodâ€™ model I think you need an integer variable, piceil(k), that describes your ceil-value in addition to your continuous pi(k) variable. The relationship is then Pi(k) =L= Piceil(k) and Piceil(k) =L= pi(k)+1. The cost is that you must use a MIP or MINLP solver (depending on whether there are other nonlinearities in your model).

Good luck

Arne

\

Arne Stolbjerg Drud

ARKI Consulting & Development A/S

Bagsvaerdvej 246A, DK-2880 Bagsvaerd, Denmark

Phone: (+45) 44 49 03 23, Fax: (+45) 44 49 03 33, email: adrud@arki.dk

From: gamsworld@googlegroups.com [mailto:gamsworld@googlegroups.com] On Behalf Of Maria Afshari Rad

Sent: Thursday, November 21, 2013 12:02 PM

To: gamsworld@googlegroups.com

Subject: Emergency

Dear members

have a model in which I have to use ceil function in one of my constraints. but when I use DNLP solvers it does not result in optimal solution. When I tested my constraints I figured out that function ceil does not operate correctly. More precisely I have a positive variable pi(k) in the constraint:

Fk(k)… sum(n $(ord(n)=t), z(k,n)) - z(k,k)-ceil(pi(k))+pi(k)=g=1;

but in .lst file I have:

Fk(n0)… (1)*pi(n0) - z(n0,n0) + z(n0,n11) =G= 1 ; (LHS = 0, INFES = 1 ****)

.

.

.

Fk(n10)… (1)*pi(n10) - z(n10,n10) + z(n10,n11) =G= 1 ;

I can’t understand why infeasibility of the constraint is 1. Can anyone help me?

Maria

–

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