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Person# Gergely Odor

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Satvik Mehul Mashkaria, Gergely Odor, Patrick Thiran

The metric dimension (MD) of a graph is a combinatorial notion capturing the minimum number of landmark nodes needed to distinguish every pair of nodes in the graph based on graph distance. We study how much the MD can increase if we add a single edge to the graph. The extra edge can either be selected adversarially, in which case we are interested in the largest possible value that the MD can take, or uniformly at random, in which case we are interested in the distribution of the MD. The adversarial setting has already been studied by [Eroh et. al., 2015] for general graphs, who found an example where the MD doubles on adding a single edge. By constructing a different example, we show that this increase can be as large as exponential. However, we believe that such a large increase can occur only in specially constructed graphs, and that in most interesting graph families, the MD at most doubles on adding a single edge. We prove this for $d$-dimensional grid graphs, by showing that $2d$ appropriately chosen corners and the endpoints of the extra edge can distinguish every pair of nodes, no matter where the edge is added. For the special case of $d=2$, we show that it suffices to choose the four corners as landmarks. Finally, when the extra edge is sampled uniformly at random, we conjecture that the MD of 2-dimensional grids converges in probability to $3+\mathrm{Ber}(8/27)$, and we give an almost complete proof.

2022Understanding epidemic propagation in large networks is an important but challenging task, especially since we usually lack information, and the information that we have is often counter-intuitive. An illustrative example is the dependence of the final size of the epidemic on the location of the initial infected agents (sources): common sense dictates that the most dangerous location for the sources is the largest city, but the second chapter of the thesis shows that this holds true only if the epidemic is just above the infection threshold.Identifying the initial infected agents can help us better understand the epidemic. The focus of this thesis is on identifying the very first infected agent, also called the source or patient zero. According to the standard assumptions, a few agents reveal their symptom onset time, and then it is our goal to identify the source based on this information, together with full knowledge of the underlying network. Unfortunately, even if we can choose the set of agents that are queried about their symptom onset time, the number of queries required for reliable source identification is too large for practical applications. In this thesis, we carefully assess if this issue can be mitigated by introducing adaptivity to the standard assumptions. Our main goal is to study the reduction in the query complexity if the queries can be chosen adaptively to previous answers, but we also investigate whether adaptively querying the edges can relax the full knowledge assumption on the network.Providing rigorous proofs about source identification with time queries is difficult. A notable exception is when the infection is passed with a known, deterministic delay from each agent to all of its neighbors, in which case the number of required non-adaptive and adaptive queries are equivalent to well-known notions in combinatorics; the metric dimension (MD) and the sequential metric dimension (SMD), respectively. We extend previous results in the field by computing the MD of a large class of random trees, where adaptivity can significantly reduce the query complexity, and the SMD of Erdos-Rényi random networks, where the reduction is found to be small, at most a constant factor. We address the case of non-deterministic diffusion processes for the first time in the mathematical literature: on the path graph, we observe a striking, double logarithmic decrease in adaptive query complexity compared to the non-adaptive case.Our analysis on the robustness of the MD to adding a single edge to specially constructed and d-dimensional grid networks suggests that even small changes in the network could easily derail source identification algorithms. This is concerning since it is difficult to obtain a perfect dataset about the entire contact network in practice. Inspired by recent implementations of contact tracing, we propose new source identification assumptions, where not only the symptom onset times, but also the edges of the network are queried by the algorithm, resulting in less, but potentially higher quality information. We propose two local search algorithms that outperform state of the art identification algorithms tailored to the new assumptions, and we analytically approximate their success probabilities on realistic random graph models. The adaptive assumptions enable us to evaluate our algorithms on a COVID-19 epidemic simulator: the first time that source identification algorithms are tested on such a complex dataset.

Victor Cyril L Lecomte, Gergely Odor, Patrick Thiran

We study the problem of identifying the source of a stochastic diffusion process spreading on a graph based on the arrival times of the diffusion at a few queried nodes. In a graph $G=(V,E)$, an unknown source node $v^* \in V$ is drawn uniformly at random, and unknown edge weights $w(e)$ for $e\in E$, representing the propagation delays along the edges, are drawn independently from a Gaussian distribution of mean $1$ and variance $\sigma^2$. An algorithm then attempts to identify $v^*$ by querying nodes $q \in V$ and being told the length of the shortest path between $q$ and $v^*$ in graph $G$ weighted by $w$. We consider two settings: \emph{non-adaptive}, in which all query nodes must be decided in advance, and \emph{adaptive}, in which each query can depend on the results of the previous ones. Both settings are motivated by an application of the problem to epidemic processes (where the source is called patient zero), which we discuss in detail. We characterize the query complexity when $G$ is an $n$-node path. In the non-adaptive setting, $\Theta(n\sigma^2)$ queries are needed for $\sigma^2 \leq 1$, and $\Theta(n)$ for $\sigma^2 \geq 1$. In the adaptive setting, somewhat surprisingly, only $\Theta(\log\log_{1/\sigma}n)$ are needed when $\sigma^2 \leq 1/2$, and $\Theta(\log \log n)+O_\sigma(1)$ when $\sigma^2 \geq 1/2$. This is the first mathematical study of source identification with time queries in a non-deterministic diffusion process.

2022