Monday, May 4, 2015
Technische Universität Berlin
Institut für Mathematik
Straße des 17. Juni 136
10623 Berlin
room MA 041
Lecture - 14:15
Abstract:
In the last years the vertex enumeration problem of polyhedra has seen a revival in the study of metabolic
networks, which increased the demand for efficient vertex enumeration algorithms for high-dimensional
polyhedra given by equalities and inequalities. The complexity of enumeration of vertices of polytopes (bounded
polyhedral) is a famous open problem in discrete and computational geometry.
In this paper we do not solve this problem, but present a type of fixed parameter tractable result.
We apply the concept of branch-decomposition to the vertex enumeration problem of polyhedra
P = {x : Sx = b; x >= 0}. For that purpose, we introduce the concept of k-
module and show how it relates to the separators of the linear matroid generated by the columns of S. This
then translates structural properties of the matroidal branch-decomposition to the context of polyhedra. We
then use this to present a total polynomial time algorithm for polytopes P for which the branch-width of
the linear matroid generated by S is bounded by a constant k.
This paper is joint work with Arne Reimers.
Colloquium - 16:00
Abstract:
We consider the online machine minimization problem in which jobs with hard deadlines arrive online over time at their release dates. The task is to determine a feasible schedule on a minimum number of machines. Our main result is a general O(m^2log m)-competitive algorithm for the preemptive online problem, where m is the optimal number of machines used in an offline solution. This is the first improvement on an O(log (p_{max}/p_{min}))-competitive algorithm by Phillips et al. (STOC 1997).
Our algorithm is constant-competitive if the offline optimum m is bounded by a constant. To develop the algorithm, we investigate two complementary special cases of the problem, namely, laminar instances and agreeable instances, for which we provide an O(log m)-competitive and an O(1)-competitive algorithm, respectively. Our O(1)-competitive algorithm for agreeable instances actually produces a non-preemptive schedule, which is of its own interest as there exists a strong lower bound of n for the general non-preemptive online machine minimization problem by Saha (FSTTCS 2013), which even holds for laminar instances. (Joint work with Kevin Schewior and Nicole Megow).