3 Search Results for "Giachoudis, Nikos"

Document
Brief Announcement
DOI: 10.4230/LIPIcs.SAND.2025.17
Brief Announcement: Broadcast via Mobile Agents in a Dynamic Network: Interplay of Graph Properties & Agents

Authors: William K. Moses Jr., Amanda Redlich, and Frederick Stock

Published in: LIPIcs, Volume 330, 4th Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2025)

Abstract
In this paper, we revisit the problem of Broadcast, introduced by Das, Giachoudis, Luccio, and Markou [OPODIS, 2020], where k+1 agents are initially placed on an n node dynamic graph, where 1 agent has a message that must be broadcast to the remaining k ignorant agents. The original paper studied the relationship between the number of agents needed to solve the problem and the edge density of the graph. The paper presented strong evidence that edge density of a graph, or the number of redundant edges within the graph, may be the correct graph property to accurately differentiate whether k = o(n) agents (low edge density) or k = Ω(n) agents (high edge density) are needed to solve the problem. In this paper, we show that surprisingly, edge density is not in fact the differentiating property. The original paper presents graphs with edge density 1.1 ̅{6} that require Ω(n) agents, however, we construct graphs with edge density > 1.1 ̅{6} and develop an algorithm to solve the problem on those graphs using only o(n) agents. We further show that the relationship between edge density and number of agents is fairly weak by first constructing graphs with edge density tending to 1 from above that require Ω(n/f(n)) agents to solve, for any function f(n) → ∞ as n → ∞. We then construct an infinite family of graphs with edge density < ρ requiring exactly k ignorant agents to solve Broadcast, for any k > 0 and ρ > 1. Finally, we investigate different versions of connectivity as possible properties determining the number of agents. We show that edge connectivity is not related to the number of agents: it is possible for a graph to have low (constant) edge connectivity but require a high (linear) number of agents to solve Broadcast. More generally, for any arbitrary λ, we construct a family of graphs on n nodes with edge connectivity (n-1)/λ requiring exactly k = n - 2 λ + 1 agents. On the other hand, we show vertex connectivity can impact the required number of agents: for any graph with vertex connectivity ≥ 3 and minimum degree δ, k ≥ δ - 1 agents are required to solve Broadcast. Finally, we show that for any graph containing a certain type of bond with m edges, Broadcast is unsolvable when k ≤ m-2.
Cite as

William K. Moses Jr., Amanda Redlich, and Frederick Stock. Brief Announcement: Broadcast via Mobile Agents in a Dynamic Network: Interplay of Graph Properties & Agents. In 4th Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 330, pp. 17:1-17:5, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{mosesjr._et_al:LIPIcs.SAND.2025.17,
  author =	{Moses Jr., William K. and Redlich, Amanda and Stock, Frederick},
  title =	{{Brief Announcement: Broadcast via Mobile Agents in a Dynamic Network: Interplay of Graph Properties \& Agents}},
  booktitle =	{4th Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2025)},
  pages =	{17:1--17:5},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-368-3},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{330},
  editor =	{Meeks, Kitty and Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SAND.2025.17},
  URN =		{urn:nbn:de:0030-drops-230708},
  doi =		{10.4230/LIPIcs.SAND.2025.17},
  annote =	{Keywords: Mobile agents, mobile robots, broadcast, dynamic graph, dynamic network}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2022.19
Evacuation from a Disk for Robots with Asymmetric Communication

Authors: Konstantinos Georgiou, Nikos Giachoudis, and Evangelos Kranakis

Published in: LIPIcs, Volume 248, 33rd International Symposium on Algorithms and Computation (ISAAC 2022)

Abstract
We consider evacuation of two robots from an Exit placed at an unknown location on the perimeter of a unit (radius) disk. The robots can move with max speed 1 and start at the center of the disk at the same time. We consider a new communication model, known as the SR model, in which the robots have communication faults as follows: one of the robots is a Sender and can only send wirelessly at any distance, while the other is a Receiver in that it can only receive wirelessly from any distance. The communication status of each robot is known to the other robot. In addition, both robots can exchange messages when they are co-located, which is known as Face-to-Face (F2F) model. There have been several studies in the literature concerning the evacuation time when both robots may employ either F2F or Wireless (WiFi) communication. The SR communication model diverges from these two in that the two robots themselves have differing communication capabilities. We study the evacuation time, namely the time it takes until the last robot reaches the Exit, and show that the evacuation time in the SR model is strictly between the F2F and the WiFi models. The main part of our technical contribution is also an evacuation algorithm in which two cooperating robots accomplish the task in worst-case time at most π+2. Interesting features of the proposed algorithm are the asymmetry inherent in the resulting trajectories, as well as that the robots do not move at full speed for the entire duration of their trajectories.
Cite as

Konstantinos Georgiou, Nikos Giachoudis, and Evangelos Kranakis. Evacuation from a Disk for Robots with Asymmetric Communication. In 33rd International Symposium on Algorithms and Computation (ISAAC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 248, pp. 19:1-19:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{georgiou_et_al:LIPIcs.ISAAC.2022.19,
  author =	{Georgiou, Konstantinos and Giachoudis, Nikos and Kranakis, Evangelos},
  title =	{{Evacuation from a Disk for Robots with Asymmetric Communication}},
  booktitle =	{33rd International Symposium on Algorithms and Computation (ISAAC 2022)},
  pages =	{19:1--19:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-258-7},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{248},
  editor =	{Bae, Sang Won and Park, Heejin},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2022.19},
  URN =		{urn:nbn:de:0030-drops-173047},
  doi =		{10.4230/LIPIcs.ISAAC.2022.19},
  annote =	{Keywords: Communication, Cycle, Evacuation, Receiver, Sender, Mobile Agents}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2020.24
Broadcasting with Mobile Agents in Dynamic Networks

Authors: Shantanu Das, Nikos Giachoudis, Flaminia L. Luccio, and Euripides Markou

Published in: LIPIcs, Volume 184, 24th International Conference on Principles of Distributed Systems (OPODIS 2020)

Abstract
We study the standard communication problem of broadcast for mobile agents moving in a network. The agents move autonomously in the network and can communicate with other agents only when they meet at a node. In this model, broadcast is a communication primitive for information transfer from one agent, the source, to all other agents. Previous studies of this problem were restricted to static networks while, in this paper, we consider the problem in dynamic networks modelled as an evolving graph. The dynamicity of the graph is unknown to the agents; in each round an adversary selects which edges of the graph are available, and an agent can choose to traverse one of the available edges adjacent to its current location. The only restriction on the adversary is that the subgraph of available edges in each round must span all nodes; in other words the evolving graph is constantly connected. The agents have global visibility allowing them to see the location of other agents in the graph and move accordingly. Depending on the topology of the underlying graph, we determine how many agents are necessary and sufficient to solve the broadcast problem in dynamic networks. While two agents plus the source are sufficient for ring networks, much larger teams of agents are necessary for denser graphs such as grid graphs and hypercubes, and finally for complete graphs of n nodes at least n-2 agents plus the source are necessary and sufficient. We show lower bounds on the number of agents and provide some algorithms for solving broadcast using the minimum number of agents, for various topologies.
Cite as

Shantanu Das, Nikos Giachoudis, Flaminia L. Luccio, and Euripides Markou. Broadcasting with Mobile Agents in Dynamic Networks. In 24th International Conference on Principles of Distributed Systems (OPODIS 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 184, pp. 24:1-24:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{das_et_al:LIPIcs.OPODIS.2020.24,
  author =	{Das, Shantanu and Giachoudis, Nikos and Luccio, Flaminia L. and Markou, Euripides},
  title =	{{Broadcasting with Mobile Agents in Dynamic Networks}},
  booktitle =	{24th International Conference on Principles of Distributed Systems (OPODIS 2020)},
  pages =	{24:1--24:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-176-4},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{184},
  editor =	{Bramas, Quentin and Oshman, Rotem and Romano, Paolo},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2020.24},
  URN =		{urn:nbn:de:0030-drops-135095},
  doi =		{10.4230/LIPIcs.OPODIS.2020.24},
  annote =	{Keywords: Distributed Algorithm, Dynamic Networks, Mobile Agents, Broadcast, Constantly Connected, Global visibility}
}
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