I want to write this expression but I know that is no possible because I sum j and for all j. I think It would have 2 indexes like j ,j´ but I don’t know how to do it gams.
Please, Can you help me with this? I find something with alias but I dont know how to use.

Thanks a lot Renger , but the error continues: “Uncontrolled set entered as constant”
I had change:
alias(j,jj);
eq_margen_1(j) … (sum(p,is(j,p))+ sum(i,(th(j,i)*Y(i,j))) - sum(k,dm(j,k))) =G= *(sum(jj,(sum(p,is(jj,p))+ sum(i,(th(jj,i)Y(i,jj))) - sum(k,dm(jj,k)))))V_NEG(dm(jj,k)/(sum(jj,dm(jj,k))));

Hi, the problem is in set k at the end of the equation: …V_NEG*dm(j,k)/sum(jj,dm(jj,k));

This is the equation exactly like the one in the image:
eq_margen_1(j) … sum(p,is(j,p))+ sum(i,th(j,i)*Y(i,j)) - sum(k,dm(j,k)) =G= sum(jj, sum(p,is(jj,p))+ sum(i,th(jj,i)*Y(i,jj)) - sum(k,dm(jj,k)))V_NEGdm(j,k)/sum(jj,dm(jj,k));

As you can see the set k is not under control.
Bye!