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### FPT-Algorithms for Computing Gromov-Hausdorff and Interleaving Distances Between Trees

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### Abstract

The Gromov-Hausdorff distance is a natural way to measure the distortion between two metric spaces. However, there has been only limited algorithmic development to compute or approximate this distance. We focus on computing the Gromov-Hausdorff distance between two metric trees. Roughly speaking, a metric tree is a metric space that can be realized by the shortest path metric on a tree. Any finite tree with positive edge weight can be viewed as a metric tree where the weight is treated as edge length and the metric is the induced shortest path metric in the tree. Previously, Agarwal et al. showed that even for trees with unit edge length, it is NP-hard to approximate the Gromov-Hausdorff distance between them within a factor of 3. In this paper, we present a fixed-parameter tractable (FPT) algorithm that can approximate the Gromov-Hausdorff distance between two general metric trees within a multiplicative factor of 14.
Interestingly, the development of our algorithm is made possible by a connection between the Gromov-Hausdorff distance for metric trees and the interleaving distance for the so-called merge trees. The merge trees arise in practice naturally as a simple yet meaningful topological summary (it is a variant of the Reeb graphs and contour trees), and are of independent interest. It turns out that an exact or approximation algorithm for the interleaving distance leads to an approximation algorithm for the Gromov-Hausdorff distance. One of the key contributions of our work is that we re-define the interleaving distance in a way that makes it easier to develop dynamic programming approaches to compute it. We then present a fixed-parameter tractable algorithm to compute the interleaving distance between two merge trees exactly, which ultimately leads to an FPT-algorithm to approximate the Gromov-Hausdorff distance between two metric trees. This exact FPT-algorithm to compute the interleaving distance between merge trees is of interest itself, as it is known that it is NP-hard to approximate it within a factor of 3, and previously the best known algorithm has an approximation factor of O(sqrt{n}) even for trees with unit edge length.

### BibTeX - Entry

```@InProceedings{farahbakhshtouli_et_al:LIPIcs:2019:11204,
author =	{Elena Farahbakhsh Touli and Yusu Wang},
title =	{{FPT-Algorithms for Computing Gromov-Hausdorff and Interleaving Distances Between Trees}},
booktitle =	{27th Annual European Symposium on Algorithms (ESA 2019)},
pages =	{83:1--83:14},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-124-5},
ISSN =	{1868-8969},
year =	{2019},
volume =	{144},
editor =	{Michael A. Bender and Ola Svensson and Grzegorz Herman},
publisher =	{Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik},
address =	{Dagstuhl, Germany},
URL =		{http://drops.dagstuhl.de/opus/volltexte/2019/11204},
URN =		{urn:nbn:de:0030-drops-112048},
doi =		{10.4230/LIPIcs.ESA.2019.83},
annote =	{Keywords: Gromov-Hausdorff distance, Interleaving distance, Merge trees}
}
```

 Keywords: Gromov-Hausdorff distance, Interleaving distance, Merge trees Seminar: 27th Annual European Symposium on Algorithms (ESA 2019) Issue date: 2019 Date of publication: 06.09.2019

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