> I know that the example has no feasible integer points, that is the > point of the example.
Unbounded integer variables cause some problems. For example, due to finite precision sometimes it is difficult to determine whether a mip instance is infeasible (like yours) or it has a huge integer feasible solution. For this reason many mip solvers (not glpk) do not allow unbounded integer variables. However, for instances that appear on practice it is always possible to find eligible estimation for bounds of all integer variables. Moreover, if you expect that in optimal point some integer variable takes on a value greater than 100, in most cases you may declare it continuous rather than integer, since the error will be less than 1%. > The problem is that due to the bug, glpsol never > even attempts to try to solve the problem with the simplex method. Glpk mip solver perform preprocessing first. The simplex method applied later to find optimal solution to lp relaxation of the preprocessed mip. > I > would like to be able to present this example to my students and let > them use glpsol to try to solve it and explain what the problem may > be, but since glpsol does not even get past preprocessing it is a > little to hard to explain what the program does. Sorry, glpk is not perfect. Why not to use the option --nointopt to disable preprocessing? > > What I can do is to change the lower bound on x1 to 0 instead of 1. > Then glpsol reports that the problem has unbounded solution (when > solving the initial LP-problem) and in the solution output it reports > status "INTEGER UNDEFINED". This change seems to not trigger the > preprocessing bug. _______________________________________________ Help-glpk mailing list [email protected] http://lists.gnu.org/mailman/listinfo/help-glpk
