We constrain demand variables d v by: The right hand side sums the number of selected edges entering v, and will therefore be either 0 or 1.
next we add variables a v and b v constrained by the equations: note that a v sums over f values on all edges entering v. by the previous constraints those f -values can only be nonzero on the (at most one) selected edge entering v. So, a v is simply the fvalue on the selected edge entering v, if one exists, and 0 otherwise.