# Mittagsseminar (in cooperation with M. Ghaffari, A. Steger and B. Sudakov)

__Mittagsseminar Talk Information__ | |

**Date and Time**: Thursday, April 30, 2009, 12:15 pm

**Duration**: This information is not available in the database

**Location**: CAB G51

**Speaker**: Bernd Gärtner

## K-Matrix Linear Complementarity Problems and Unique Sink Orientations

The linear complementarity problem (LCP) is the following: given a
square matrix M and a vector q, find nonnegative vectors w and z such
that w - Mz = q and w^T z = 0. The question whether such w and z exist
is NP-complete in general. If M is a P-matrix (all principal minors
are positive), there are unique solution vectors w and z, but the
computational complexity of finding them is unknown.

On a combinatorial level, the P-matrix LCP can be studied using the
concept of unique sink orientations of cubes (USO). Recent research
has focused on the translation of algebraic properties of the matrix
M into combinatorial properties of the underlying USO. We will report
on one result along these lines. If M is a K-matrix (a P-matrix such
that all off-diagonal entries are nonpositive), the underlying USO
contains short directed paths only. This generalizes (and at the same
time combinatorially explains) the known result that K-matrix LCPs are
polynomial-time solvable.

Joint work with Jan Foniok, Komei Fukuda and Hans-Jakob Lüthi.

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