8,853 research outputs found
Intersection disjunctions for reverse convex sets
We present a framework to obtain valid inequalities for optimization problems
constrained by a reverse convex set, which is defined as the set of points in a
polyhedron that lie outside a given open convex set. We are particularly
interested in cases where the closure of the convex set is either
non-polyhedral, or is defined by too many inequalities to directly apply
disjunctive programming. Reverse convex sets arise in many models, including
bilevel optimization and polynomial optimization. Intersection cuts are a
well-known method for generating valid inequalities for a reverse convex set.
Intersection cuts are generated from a basic solution that lies within the
convex set. Our contribution is a framework for deriving valid inequalities for
the reverse convex set from basic solutions that lie outside the convex set. We
begin by proposing an extension to intersection cuts that defines a two-term
disjunction for a reverse convex set. Next, we generalize this analysis to a
multi-term disjunction by considering the convex set's recession directions.
These disjunctions can be used in a cut-generating linear program to obtain
disjunctive cuts for the reverse convex set.Comment: 24 page
Submodular Minimization Under Congruency Constraints
Submodular function minimization (SFM) is a fundamental and efficiently
solvable problem class in combinatorial optimization with a multitude of
applications in various fields. Surprisingly, there is only very little known
about constraint types under which SFM remains efficiently solvable. The
arguably most relevant non-trivial constraint class for which polynomial SFM
algorithms are known are parity constraints, i.e., optimizing only over sets of
odd (or even) cardinality. Parity constraints capture classical combinatorial
optimization problems like the odd-cut problem, and they are a key tool in a
recent technique to efficiently solve integer programs with a constraint matrix
whose subdeterminants are bounded by two in absolute value.
We show that efficient SFM is possible even for a significantly larger class
than parity constraints, by introducing a new approach that combines techniques
from Combinatorial Optimization, Combinatorics, and Number Theory. In
particular, we can show that efficient SFM is possible over all sets (of any
given lattice) of cardinality r mod m, as long as m is a constant prime power.
This covers generalizations of the odd-cut problem with open complexity status,
and with relevance in the context of integer programming with higher
subdeterminants. To obtain our results, we establish a connection between the
correctness of a natural algorithm, and the inexistence of set systems with
specific combinatorial properties. We introduce a general technique to disprove
the existence of such set systems, which allows for obtaining extensions of our
results beyond the above-mentioned setting. These extensions settle two open
questions raised by Geelen and Kapadia [Combinatorica, 2017] in the context of
computing the girth and cogirth of certain types of binary matroids
On optimizing over lift-and-project closures
The lift-and-project closure is the relaxation obtained by computing all
lift-and-project cuts from the initial formulation of a mixed integer linear
program or equivalently by computing all mixed integer Gomory cuts read from
all tableau's corresponding to feasible and infeasible bases. In this paper, we
present an algorithm for approximating the value of the lift-and-project
closure. The originality of our method is that it is based on a very simple cut
generation linear programming problem which is obtained from the original
linear relaxation by simply modifying the bounds on the variables and
constraints. This separation LP can also be seen as the dual of the cut
generation LP used in disjunctive programming procedures with a particular
normalization. We study some properties of this separation LP in particular
relating it to the equivalence between lift-and-project cuts and Gomory cuts
shown by Balas and Perregaard. Finally, we present some computational
experiments and comparisons with recent related works
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