# Temporal Matching

## TL;DR

This paper introduces the concept of temporal matching in link streams, proves its computational hardness, and provides practical algorithms with real-world and simulated data evaluations.

## Contribution

It defines temporal matching, proves NP-hardness for b3>1, and offers kernelization and approximation algorithms with open-source implementations.

## Key findings

- Maximum temporal matching computation is NP-hard for b3>1.
- The 2-approximation algorithm performs well on real and simulated data.
- Kernelization reduces problem size effectively in practical scenarios.

## Abstract

A link stream is a sequence of pairs of the form $(t,\{u,v\})$, where $t\in\mathbb N$ represents a time instant and $u\neq v$. Given an integer $\gamma$, the $\gamma$-edge between vertices $u$ and $v$, starting at time $t$, is the set of temporally consecutive edges defined by $\{(t',\{u,v\}) | t' \in [t,t+\gamma-1]\}$. We introduce the notion of temporal matching of a link stream to be an independent $\gamma$-edge set belonging to the link stream. We show that the problem of computing a temporal matching of maximum size is NP-hard as soon as $\gamma>1$. We depict a kernelization algorithm parameterized by the solution size for the problem. As a byproduct we also give a $2$-approximation algorithm. Both our $2$-approximation and kernelization algorithms are implemented and confronted to link streams collected from real world graph data. We observe that finding temporal matchings is a sensitive question when mining our data from such a perspective as: managing peer-working when any pair of peers $X$ and $Y$ are to collaborate over a period of one month, at an average rate of at least two email exchanges every week. We furthermore design a link stream generating process by mimicking the behaviour of a random moving group of particles under natural simulation, and confront our algorithms to these generated instances of link streams. All the implementations are open source.