## Theory of Combinatorial Algorithms

Prof. Emo Welzl and Prof. Bernd Gärtner

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

 Mittagsseminar Talk Information

Date and Time: Thursday, December 17, 2015, 12:15 pm

Duration: 30 minutes

Location: CAB G51

Speaker: Vincent Kusters

## The planar tree packing theorem

Packing graphs is a combinatorial problem where several given graphs are being mapped into a common host graph such that every edge is used at most once. In the planar tree packing problem we are given two trees $T_1$ and $T_2$ on $n$ vertices and have to find a planar graph on $n$ vertices that is the edge-disjoint union of $T_1$ and $T_2$. A clear exception that must be made is the star which cannot be packed together with any other tree. But according to a conjecture of Garcia et al from 1997 this is the only exception, and all other pairs of trees admit a planar packing. Previous results addressed various special cases, such as a tree and a spider tree, a tree and a caterpillar, two trees of diameter four, two isomorphic trees, and trees of maximum degree three. Here we settle the conjecture in the affirmative and prove its general form, thus making it the planar tree packing theorem. The proof is constructive and provides a polynomial time algorithm to obtain a packing for two given non-star trees.

Joint work with Markus Geyer, Micheal Hoffmann, Micheal Kaufmann, and Csaba Toth.

Information for students and suggested topics for student talks