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Hi Bussieck,
Thanks for your kindly reply my problem.
It helped me much.

Regards.
Huseyin

Thanks for you reply. It means if i=1 then l=1. By the way, i aliases j. When i=1, j can not be equal to 1. Or if j=1 then k=1. The same here, if j =1, i can not be equal to 1. So, here is the question. How can i define valid_y. Should it be under the variable header ? or under the parameter header ...

Hello, I need a help for my GAMS code. I have sets i and l ; alias(i,j) and alias (l,k). Also i have variable ; y(i,l,j,k). I want to declare valid_y as in the picture and use it in the following constraints. Adsız.jpg WhatsApp Image 2022-06-19 at 16.03.00 (1).jpeg con2(j).. sum((i,l,k), y(i,l,j,k))...