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Papers by Evangelos Bampas
Lecture Notes in Computer Science, 2015
Lecture Notes in Computer Science, 2015
We address the problem of verifying the accuracy of a map of a network by making as few measureme... more We address the problem of verifying the accuracy of a map of a network by making as few measurements as possible on its nodes. This task can be formalized as an optimization problem that, given a graph G=(V, E), and a query model specifying the information returned by a query at a node, asks for finding a minimum-size subset of nodes of G to be queried so as to univocally identify G. This problem has been faced wrt a couple of query models assuming that a node had some global knowledge about the network. Here, we propose a ...
Cologne Twente Workshop on Graphs and Combinatorical Optimization, 2009
Cologne Twente Workshop on Graphs and Combinatorical Optimization, 2008
Wavelength Division Multiplexing (WDM) is a dominating technology in contem-porary all-optical ne... more Wavelength Division Multiplexing (WDM) is a dominating technology in contem-porary all-optical networking. It allows several connections to be established through the same fiber links, provided that each of the connections uses a different wave-length. A second requirement is that a ...
We study the problem of satisfying a maximum number of communication requests in all- optical WDM... more We study the problem of satisfying a maximum number of communication requests in all- optical WDM rings in which the number of available wavelengths per fiber is limited. We investigate two variations of the problem: with or without prior routing of requests. We consider a number of new and existing algorithmic approaches for these two variations. We perform an experimental
We investigate the complexity of hard counting problems that belong to the class #P but have easy... more We investigate the complexity of hard counting problems that belong to the class #P but have easy decision version; several wellknown problems such as #Perfect Matchings, #DNFSat share this property. We focus on classes of such problems which emerged through two disparate approaches: one taken by Hemaspaandra et al.
Abstract: Containment-based trees encompass various handy structures such as B+-trees, R-trees an... more Abstract: Containment-based trees encompass various handy structures such as B+-trees, R-trees and M-trees. They are widely used to build data indexes, range-queryable overlays, publish/subscribe systems both in centralized and distributed contexts. In addition to their versatility, their balanced shape ensures an overall satisfactory performance. Re-cently, it has been shown that their distributed implementations can be fault-resilient. However, this robustness is achieved at the cost of un-balancing the structure.
We propose and investigate Selfish Path MultiColoring games as a natural model for noncooperative... more We propose and investigate Selfish Path MultiColoring games as a natural model for noncooperative wavelength assignment in multifiber optical networks. In this setting, we view the wavelength assignment process as a strategic game in which each communication request selfishly chooses a wavelength in an effort to minimize the maximum congestion that it encounters on the chosen wavelength. We measure the cost of a certain wavelength assignment as the maximum, among all physical links, number of parallel fibers employed by this assignment. We start by settling questions related to the existence and computation of and convergence to pure Nash equilibria in these games. Our main contribution is a thorough analysis of the price of anarchy of such games, that is, the worst-case ratio between the cost of a Nash equilibrium and the optimal cost. We first provide upper bounds on the price of anarchy for games defined on general network topologies. Along the way, we obtain an upper bound of 2 for games defined on star networks. We next show that our bounds are tight even in the case of tree networks of maximum degree 3, leading to nonconstant price of anarchy for such topologies. In contrast, for network topologies of maximum degree 2, the quality of the solutions obtained by selfish wavelength assignment is much more satisfactory: We prove that the price of anarchy is bounded by 4 for a large class of practically interesting games defined on ring networks.
We address the problem of verifying the accuracy of a map of a network by making as few measureme... more We address the problem of verifying the accuracy of a map of a network by making as few measurements as possible on its nodes. This task can be formalized as an optimization problem that, given a graph G = (V,E), and a query model specifying the information returned by a query at a node, asks for finding a minimum-size subset of nodes of G to be queried so as to univocally identify G. This problem has been faced w.r.t. a couple of query models assuming that a node had some global knowledge about the network. Here, we propose a new query model based on the local knowledge a node instead usually has. Quite naturally, we assume that a query at a given node returns the associated routing table, i.e., a set of entries which provides, for each destination node, a corresponding (set of) first-hop node(s) along an underlying shortest path. First, we show that any network of n nodes needs Ω(loglogn) queries to be verified. Then, we prove that there is no o(logn)-approximation algorithm for the problem, unless \sf P=\sf NP, even for networks of diameter 2. On the positive side, we provide an O(logn)-approximation algorithm to verify a network of diameter 2, and we give exact polynomial-time algorithms for paths, trees, and cycles of even length.
We are interested in the problem of satisfying a maximum-profit subset of undirected communicatio... more We are interested in the problem of satisfying a maximum-profit subset of undirected communication requests in an optical ring that uses the Wavelength Division Multiplexing technology. We present four deterministic and purely combinatorial algorithms for this problem, and give theoretical guarantees for their worst-case approximation ratios. Two of these algorithms are novel, whereas the rest are adaptation of earlier approaches. An experimental evaluation of the algorithms in terms of attained profit and execution time reveals that the theoretically best algorithm performs only marginally better than one of the new algorithms, while at the same time being several orders of magnitude slower. Furthermore, an extremely fast greedy heuristic with nonconstant approximation ratio performs reasonably well and may be favored over the other algorithms whenever it is crucial to minimize execution time.
Two anonymous mobile agents (robots) moving in an asynchronous manner have to meet in an infinite... more Two anonymous mobile agents (robots) moving in an asynchronous manner have to meet in an infinite grid of dimension δ> 0, starting from two arbitrary positions at distance at most d. Since the problem is clearly infeasible in such general setting, we assume that the grid is embedded in a δ-dimensional Euclidean space and that each agent knows the Cartesian coordinates of its own initial position (but not the one of the other agent). We design an algorithm permitting the agents to meet after traversing a trajectory of length O(d^δ polylogd). This bound for the case of 2d -grids subsumes the main result of [12]. The algorithm is almost optimal, since the Ω(d^δ ) lower bound is straightforward.
Further, we apply our rendezvous method to the following network design problem. The ports of the δ-dimensional grid have to be set such that two anonymous agents starting at distance at most d from each other will always meet, moving in an asynchronous manner, after traversing a O(d^δ polylogd) length trajectory.
We can also apply our method to a version of the geometric rendezvous problem. Two anonymous agents move asynchronously in the δ-dimensional Euclidean space. The agents have the radii of visibility of r1 and r2, respectively. Each agent knows only its own initial position and its own radius of visibility. The agents meet when one agent is visible to the other one. We propose an algorithm designing the trajectory of each agent, so that they always meet after traveling a total distance of O(((d/r)r^δ)polylog((d/r)^δ)), where r = min (r1,r 2) and for r ≥ 1.
Lecture Notes in Computer Science, 2015
Lecture Notes in Computer Science, 2015
We address the problem of verifying the accuracy of a map of a network by making as few measureme... more We address the problem of verifying the accuracy of a map of a network by making as few measurements as possible on its nodes. This task can be formalized as an optimization problem that, given a graph G=(V, E), and a query model specifying the information returned by a query at a node, asks for finding a minimum-size subset of nodes of G to be queried so as to univocally identify G. This problem has been faced wrt a couple of query models assuming that a node had some global knowledge about the network. Here, we propose a ...
Cologne Twente Workshop on Graphs and Combinatorical Optimization, 2009
Cologne Twente Workshop on Graphs and Combinatorical Optimization, 2008
Wavelength Division Multiplexing (WDM) is a dominating technology in contem-porary all-optical ne... more Wavelength Division Multiplexing (WDM) is a dominating technology in contem-porary all-optical networking. It allows several connections to be established through the same fiber links, provided that each of the connections uses a different wave-length. A second requirement is that a ...
We study the problem of satisfying a maximum number of communication requests in all- optical WDM... more We study the problem of satisfying a maximum number of communication requests in all- optical WDM rings in which the number of available wavelengths per fiber is limited. We investigate two variations of the problem: with or without prior routing of requests. We consider a number of new and existing algorithmic approaches for these two variations. We perform an experimental
We investigate the complexity of hard counting problems that belong to the class #P but have easy... more We investigate the complexity of hard counting problems that belong to the class #P but have easy decision version; several wellknown problems such as #Perfect Matchings, #DNFSat share this property. We focus on classes of such problems which emerged through two disparate approaches: one taken by Hemaspaandra et al.
Abstract: Containment-based trees encompass various handy structures such as B+-trees, R-trees an... more Abstract: Containment-based trees encompass various handy structures such as B+-trees, R-trees and M-trees. They are widely used to build data indexes, range-queryable overlays, publish/subscribe systems both in centralized and distributed contexts. In addition to their versatility, their balanced shape ensures an overall satisfactory performance. Re-cently, it has been shown that their distributed implementations can be fault-resilient. However, this robustness is achieved at the cost of un-balancing the structure.
We propose and investigate Selfish Path MultiColoring games as a natural model for noncooperative... more We propose and investigate Selfish Path MultiColoring games as a natural model for noncooperative wavelength assignment in multifiber optical networks. In this setting, we view the wavelength assignment process as a strategic game in which each communication request selfishly chooses a wavelength in an effort to minimize the maximum congestion that it encounters on the chosen wavelength. We measure the cost of a certain wavelength assignment as the maximum, among all physical links, number of parallel fibers employed by this assignment. We start by settling questions related to the existence and computation of and convergence to pure Nash equilibria in these games. Our main contribution is a thorough analysis of the price of anarchy of such games, that is, the worst-case ratio between the cost of a Nash equilibrium and the optimal cost. We first provide upper bounds on the price of anarchy for games defined on general network topologies. Along the way, we obtain an upper bound of 2 for games defined on star networks. We next show that our bounds are tight even in the case of tree networks of maximum degree 3, leading to nonconstant price of anarchy for such topologies. In contrast, for network topologies of maximum degree 2, the quality of the solutions obtained by selfish wavelength assignment is much more satisfactory: We prove that the price of anarchy is bounded by 4 for a large class of practically interesting games defined on ring networks.
We address the problem of verifying the accuracy of a map of a network by making as few measureme... more We address the problem of verifying the accuracy of a map of a network by making as few measurements as possible on its nodes. This task can be formalized as an optimization problem that, given a graph G = (V,E), and a query model specifying the information returned by a query at a node, asks for finding a minimum-size subset of nodes of G to be queried so as to univocally identify G. This problem has been faced w.r.t. a couple of query models assuming that a node had some global knowledge about the network. Here, we propose a new query model based on the local knowledge a node instead usually has. Quite naturally, we assume that a query at a given node returns the associated routing table, i.e., a set of entries which provides, for each destination node, a corresponding (set of) first-hop node(s) along an underlying shortest path. First, we show that any network of n nodes needs Ω(loglogn) queries to be verified. Then, we prove that there is no o(logn)-approximation algorithm for the problem, unless \sf P=\sf NP, even for networks of diameter 2. On the positive side, we provide an O(logn)-approximation algorithm to verify a network of diameter 2, and we give exact polynomial-time algorithms for paths, trees, and cycles of even length.
We are interested in the problem of satisfying a maximum-profit subset of undirected communicatio... more We are interested in the problem of satisfying a maximum-profit subset of undirected communication requests in an optical ring that uses the Wavelength Division Multiplexing technology. We present four deterministic and purely combinatorial algorithms for this problem, and give theoretical guarantees for their worst-case approximation ratios. Two of these algorithms are novel, whereas the rest are adaptation of earlier approaches. An experimental evaluation of the algorithms in terms of attained profit and execution time reveals that the theoretically best algorithm performs only marginally better than one of the new algorithms, while at the same time being several orders of magnitude slower. Furthermore, an extremely fast greedy heuristic with nonconstant approximation ratio performs reasonably well and may be favored over the other algorithms whenever it is crucial to minimize execution time.
Two anonymous mobile agents (robots) moving in an asynchronous manner have to meet in an infinite... more Two anonymous mobile agents (robots) moving in an asynchronous manner have to meet in an infinite grid of dimension δ> 0, starting from two arbitrary positions at distance at most d. Since the problem is clearly infeasible in such general setting, we assume that the grid is embedded in a δ-dimensional Euclidean space and that each agent knows the Cartesian coordinates of its own initial position (but not the one of the other agent). We design an algorithm permitting the agents to meet after traversing a trajectory of length O(d^δ polylogd). This bound for the case of 2d -grids subsumes the main result of [12]. The algorithm is almost optimal, since the Ω(d^δ ) lower bound is straightforward.
Further, we apply our rendezvous method to the following network design problem. The ports of the δ-dimensional grid have to be set such that two anonymous agents starting at distance at most d from each other will always meet, moving in an asynchronous manner, after traversing a O(d^δ polylogd) length trajectory.
We can also apply our method to a version of the geometric rendezvous problem. Two anonymous agents move asynchronously in the δ-dimensional Euclidean space. The agents have the radii of visibility of r1 and r2, respectively. Each agent knows only its own initial position and its own radius of visibility. The agents meet when one agent is visible to the other one. We propose an algorithm designing the trajectory of each agent, so that they always meet after traveling a total distance of O(((d/r)r^δ)polylog((d/r)^δ)), where r = min (r1,r 2) and for r ≥ 1.