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Extremal Functions for Shortening Sets of Paths

Published online by Cambridge University Press:  14 August 2006

PAUL WOLLAN
Affiliation:
School of Mathematics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA (e-mail: wollan@math.gatech.edu)

Abstract

Let $P_1, {\ldots}\,,P_k$ be $k$ vertex-disjoint paths in a graph $G$ where the ends of $P_i$ are $x_i$, and $y_i$. Let $H$ be the subgraph induced by the vertex sets of the paths. We find edge bounds $E_1(n)$, $E_2(n)$ such that:

  1. if $e(H) \geq E_1(|V(H)|)$, then there exist disjoint paths $P_1', {\ldots}\,,P_k'$ where the ends of $P_i'$ are $x_i$ and $ y_i$ such that $|\bigcup_i V(P_i)| > |\bigcup_i V(P_i')|$;

  2. if $e(H) \geq E_2(|V(H)|)$, then there exist disjoint paths $P_1', {\ldots}\,, P_k'$ where the ends of $P_i'$ are $x_i'$ and $y_i'$ such that $|\bigcup_i V(P_i)| > |\bigcup_i V(P_i')|$ and $\{ x_1, {\ldots}\,, x_k \} = \{ x_1' , {\ldots}\, , x_k' \}$ and $\{ y_1, {\ldots}\,, y_k \} = \{ y_1', {\ldots}\,, y_k'\}$.

The bounds are the best possible, in that there exist arbitrarily large graphs $H'$ with $e(H') = E_i (H') - 1$ without the properties stipulated in 1 and 2.

Type
Paper
Copyright
2006 Cambridge University Press

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