1,128 research outputs found
Tight Lower Bounds on the Sizes of Symmetric Extensions of Permutahedra and Similar Results
It is well known that the permutahedron Pi_n has 2^n-2 facets. The Birkhoff
polytope provides a symmetric extended formulation of Pi_n of size Theta(n^2).
Recently, Goemans described a non-symmetric extended formulation of Pi_n of
size Theta(n log(n)). In this paper, we prove that Omega(n^2) is a lower bound
for the size of symmetric extended formulations of Pi_n.Comment: corrected an error in the linear description of the permutahedron in
introductio
Symmetry Matters for Sizes of Extended Formulations
In 1991, Yannakakis (J. Comput. System Sci., 1991) proved that no symmetric
extended formulation for the matching polytope of the complete graph K_n with n
nodes has a number of variables and constraints that is bounded
subexponentially in n. Here, symmetric means that the formulation remains
invariant under all permutations of the nodes of K_n. It was also conjectured
in the paper mentioned above that "asymmetry does not help much," but no
corresponding result for general extended formulations has been found so far.
In this paper we show that for the polytopes associated with the matchings in
K_n with log(n) (rounded down) edges there are non-symmetric extended
formulations of polynomial size, while nevertheless no symmetric extended
formulations of polynomial size exist. We furthermore prove similar statements
for the polytopes associated with cycles of length log(n) (rounded down). Thus,
with respect to the question for smallest possible extended formulations, in
general symmetry requirements may matter a lot. Compared to the extended
abtract that has appeared in the Proceedings of IPCO XIV at Lausanne, this
paper does not only contain proofs that had been ommitted there, but it also
presents slightly generalized and sharpened lower bounds.Comment: 24 pages; incorporated referees' comments; to appear in: SIAM Journal
on Discrete Mathematic
An extension of disjunctive programming and its impact for compact tree formulations
In the 1970's, Balas introduced the concept of disjunctive programming, which
is optimization over unions of polyhedra. One main result of his theory is
that, given linear descriptions for each of the polyhedra to be taken in the
union, one can easily derive an extended formulation of the convex hull of the
union of these polyhedra. In this paper, we give a generalization of this
result by extending the polyhedral structure of the variables coupling the
polyhedra taken in the union. Using this generalized concept, we derive
polynomial size linear programming formulations (compact formulations) for a
well-known spanning tree approximation of Steiner trees, for Gomory-Hu trees,
and, as a consequence, of the minimum -cut problem (but not for the
associated -cut polyhedron). Recently, Kaibel and Loos (2010) introduced a
more involved framework called {\em polyhedral branching systems} to derive
extended formulations. The most parts of our model can be expressed in terms of
their framework. The value of our model can be seen in the fact that it
completes their framework by an interesting algorithmic aspect.Comment: 17 page
Optimization Modulo Theories with Linear Rational Costs
In the contexts of automated reasoning (AR) and formal verification (FV),
important decision problems are effectively encoded into Satisfiability Modulo
Theories (SMT). In the last decade efficient SMT solvers have been developed
for several theories of practical interest (e.g., linear arithmetic, arrays,
bit-vectors). Surprisingly, little work has been done to extend SMT to deal
with optimization problems; in particular, we are not aware of any previous
work on SMT solvers able to produce solutions which minimize cost functions
over arithmetical variables. This is unfortunate, since some problems of
interest require this functionality.
In the work described in this paper we start filling this gap. We present and
discuss two general procedures for leveraging SMT to handle the minimization of
linear rational cost functions, combining SMT with standard minimization
techniques. We have implemented the procedures within the MathSAT SMT solver.
Due to the absence of competitors in the AR, FV and SMT domains, we have
experimentally evaluated our implementation against state-of-the-art tools for
the domain of linear generalized disjunctive programming (LGDP), which is
closest in spirit to our domain, on sets of problems which have been previously
proposed as benchmarks for the latter tools. The results show that our tool is
very competitive with, and often outperforms, these tools on these problems,
clearly demonstrating the potential of the approach.Comment: Submitted on january 2014 to ACM Transactions on Computational Logic,
currently under revision. arXiv admin note: text overlap with arXiv:1202.140
Robustness of controllers designed using Galerkin type approximations
One of the difficulties in designing controllers for infinite-dimensional systems arises from attempting to calculate a state for the system. It is shown that Galerkin type approximations can be used to design controllers which will perform as designed when implemented on the original infinite-dimensional system. No assumptions, other than those typically employed in numerical analysis, are made on the approximating scheme
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