  This paper contributes to the theory of cutting planes for mixed integer
linear programs (MILPs). Minimal valid inequalities are well understood for a
relaxation of an MILP in tableau form where all the nonbasic variables are
continuous; they are derived using the gauge function of maximal lattice-free
convex sets. In this paper we study lifting functions for the nonbasic integer
variables starting from such minimal valid inequalities. We characterize
precisely when the lifted coefficient is equal to the coefficient of the
corresponding continuous variable in every minimal lifting. The answer is a
nonconvex region that can be obtained as a finite union of convex polyhedra. We
then establish a necessary and sufficient condition for the uniqueness of the
lifting function.
