hi

I have a serious problem with solving my model in gams environment , i have written model of VRPTW in gams wich set i shows all nodes contains depot and customers and s is service time and at is arrival time. but i I was faced with an error 56 which is " endongenous operands for * not allowed in linear models " …can anyone help me on this issue???

objective … z=e=sum((i,j,k)$(ord(i) ne ord(j)),(t(i,j)*x(i,j,k))) ;

constraint1 (i) … sum((j,k)$(ord(i) ne ord(j) and not ord(j)=1),x(i,j,k))=e=1 ;

constraint2 (j) … sum((i,k)$(ord(i) ne ord(j) and not ord(i)=1),x(i,j,k))=e=1 ;

constraint3 (‘depot’) … sum((j,k)(not ord(j)=1),x('depot',j,k))=e=sum((j,k)(not ord(j)=1),x(j,‘depot’,k)) ;

constraint4 (‘depot’) … sum((j,k)$(not ord(j)=1),x(‘depot’,j,k))=l=1 ;

constraint5 (‘depot’) … sum((j,k)$(not ord(j)=1),x(‘depot’,j,k))=l=1 ;

constraint6 (k)… sum(i,q(i)*(sum(j,x(i,j,k))))=e=900 ;

constraint7 (i,j,k) … (x(i,j,k))*(at(i,k)+s(i)+t(i,j)-at(j,k))=e=0 ;

constraint8 (i,k) … e(i)=l=at(i,k) ;

constraint9 (i,k) … at(i,k)=l=u(i) ;

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