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Documents authored by Forster, Sebastian

Document
Track A: Algorithms, Complexity and Games
DOI: 10.4230/LIPIcs.ICALP.2024.58
New Tradeoffs for Decremental Approximate All-Pairs Shortest Paths

Authors: Michal Dory, Sebastian Forster, Yasamin Nazari, and Tijn de Vos

Published in: LIPIcs, Volume 297, 51st International Colloquium on Automata, Languages, and Programming (ICALP 2024)

Abstract
We provide new tradeoffs between approximation and running time for the decremental all-pairs shortest paths (APSP) problem. For undirected graphs with m edges and n nodes undergoing edge deletions, we provide four new approximate decremental APSP algorithms, two for weighted and two for unweighted graphs. Our first result is (2+ε)-APSP with total update time Õ(m^{1/2}n^{3/2}) (when m = n^{1+c} for any constant 0 < c < 1). Prior to our work the fastest algorithm for weighted graphs with approximation at most 3 had total Õ(mn) update time for (1+ε)-APSP [Bernstein, SICOMP 2016]. Our second result is (2+ε, W_{u,v})-APSP with total update time Õ(nm^{3/4}), where the second term is an additive stretch with respect to W_{u,v}, the maximum weight on the shortest path from u to v. Our third result is (2+ε)-APSP for unweighted graphs in Õ(m^{7/4}) update time, which for sparse graphs (m = o(n^{8/7})) is the first subquadratic (2+ε)-approximation. Our last result for unweighted graphs is (1+ε, 2(k-1))-APSP, for k ≥ 2, with Õ(n^{2-1/k}m^{1/k}) total update time (when m = n^{1+c} for any constant c > 0). For comparison, in the special case of (1+ε, 2)-approximation, this improves over the state-of-the-art algorithm by [Henzinger, Krinninger, Nanongkai, SICOMP 2016] with total update time of Õ(n^{2.5}). All of our results are randomized, work against an oblivious adversary, and have constant query time.
Cite as

Michal Dory, Sebastian Forster, Yasamin Nazari, and Tijn de Vos. New Tradeoffs for Decremental Approximate All-Pairs Shortest Paths. In 51st International Colloquium on Automata, Languages, and Programming (ICALP 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 297, pp. 58:1-58:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{dory_et_al:LIPIcs.ICALP.2024.58,
  author =	{Dory, Michal and Forster, Sebastian and Nazari, Yasamin and de Vos, Tijn},
  title =	{{New Tradeoffs for Decremental Approximate All-Pairs Shortest Paths}},
  booktitle =	{51st International Colloquium on Automata, Languages, and Programming (ICALP 2024)},
  pages =	{58:1--58:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-322-5},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{297},
  editor =	{Bringmann, Karl and Grohe, Martin and Puppis, Gabriele and Svensson, Ola},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2024.58},
  URN =		{urn:nbn:de:0030-drops-202012},
  doi =		{10.4230/LIPIcs.ICALP.2024.58},
  annote =	{Keywords: Decremental Shortest Path, All-Pairs Shortest Paths}
}
Document
DOI: 10.4230/LIPIcs.ESA.2023.50
Bootstrapping Dynamic Distance Oracles

Authors: Sebastian Forster, Gramoz Goranci, Yasamin Nazari, and Antonis Skarlatos

Published in: LIPIcs, Volume 274, 31st Annual European Symposium on Algorithms (ESA 2023)

Abstract
Designing approximate all-pairs distance oracles in the fully dynamic setting is one of the central problems in dynamic graph algorithms. Despite extensive research on this topic, the first result breaking the O(√n) barrier on the update time for any non-trivial approximation was introduced only recently by Forster, Goranci and Henzinger [SODA'21] who achieved m^{1/ρ+o(1)} amortized update time with a O(log n)^{3ρ-2} factor in the approximation ratio, for any parameter ρ ≥ 1. In this paper, we give the first constant-stretch fully dynamic distance oracle with small polynomial update and query time. Prior work required either at least a poly-logarithmic approximation or much larger update time. Our result gives a more fine-grained trade-off between stretch and update time, for instance we can achieve constant stretch of O(1/(ρ²))^{4/ρ} in amortized update time Õ(n^{ρ}), and query time Õ(n^{ρ/8}) for any constant parameter 0 < ρ < 1. Our algorithm is randomized and assumes an oblivious adversary. A core technical idea underlying our construction is to design a black-box reduction from decremental approximate hub-labeling schemes to fully dynamic distance oracles, which may be of independent interest. We then apply this reduction repeatedly to an existing decremental algorithm to bootstrap our fully dynamic solution.
Cite as

Sebastian Forster, Gramoz Goranci, Yasamin Nazari, and Antonis Skarlatos. Bootstrapping Dynamic Distance Oracles. In 31st Annual European Symposium on Algorithms (ESA 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 274, pp. 50:1-50:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{forster_et_al:LIPIcs.ESA.2023.50,
  author =	{Forster, Sebastian and Goranci, Gramoz and Nazari, Yasamin and Skarlatos, Antonis},
  title =	{{Bootstrapping Dynamic Distance Oracles}},
  booktitle =	{31st Annual European Symposium on Algorithms (ESA 2023)},
  pages =	{50:1--50:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-295-2},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{274},
  editor =	{G{\o}rtz, Inge Li and Farach-Colton, Martin and Puglisi, Simon J. and Herman, Grzegorz},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2023.50},
  URN =		{urn:nbn:de:0030-drops-187031},
  doi =		{10.4230/LIPIcs.ESA.2023.50},
  annote =	{Keywords: Dynamic graph algorithms, Distance Oracles, Shortest Paths}
}
Document
DOI: 10.4230/DagRep.12.11.45
Dynamic Graph Algorithms (Dagstuhl Seminar 22461)

Authors: Aaron Bernstein, Shiri Chechik, Sebastian Forster, Tsvi Kopelowitz, Yasamin Nazari, and Nicole Wein

Published in: Dagstuhl Reports, Volume 12, Issue 11 (2023)

Abstract
This report documents the program and the outcomes of Dagstuhl Seminar 22461 “Dynamic Graph Algorithms”, which took place from November 13 to November 18, 2022. The field of dynamic graph algorithms studies algorithms for processing graphs that are changing over time. Formally, the goal is to process an interleaved sequence of update and query operations, where an update operation changes the input graph (e.g. inserts/deletes an edge), while the query operation is problem-specific and asks for some information about the current graph – for example, an s-t path, or a minimum spanning tree. The field has evolved rapidly over the past decade, and this Dagstuhl Seminar brought together leading researchers in dynamic algorithms and related areas of graph algorithms.
Cite as

Aaron Bernstein, Shiri Chechik, Sebastian Forster, Tsvi Kopelowitz, Yasamin Nazari, and Nicole Wein. Dynamic Graph Algorithms (Dagstuhl Seminar 22461). In Dagstuhl Reports, Volume 12, Issue 11, pp. 45-65, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@Article{bernstein_et_al:DagRep.12.11.45,
  author =	{Bernstein, Aaron and Chechik, Shiri and Forster, Sebastian and Kopelowitz, Tsvi and Nazari, Yasamin and Wein, Nicole},
  title =	{{Dynamic Graph Algorithms (Dagstuhl Seminar 22461)}},
  pages =	{45--65},
  journal =	{Dagstuhl Reports},
  ISSN =	{2192-5283},
  year =	{2023},
  volume =	{12},
  number =	{11},
  editor =	{Bernstein, Aaron and Chechik, Shiri and Forster, Sebastian and Kopelowitz, Tsvi and Nazari, Yasamin and Wein, Nicole},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DagRep.12.11.45},
  URN =		{urn:nbn:de:0030-drops-178354},
  doi =		{10.4230/DagRep.12.11.45},
  annote =	{Keywords: dynamic graphs, graph algorithms}
}
Document
Track A: Algorithms, Complexity and Games
DOI: 10.4230/LIPIcs.ICALP.2022.61
Faster Cut Sparsification of Weighted Graphs

Authors: Sebastian Forster and Tijn de Vos

Published in: LIPIcs, Volume 229, 49th International Colloquium on Automata, Languages, and Programming (ICALP 2022)

Abstract
A cut sparsifier is a reweighted subgraph that maintains the weights of the cuts of the original graph up to a multiplicative factor of (1±ε). This paper considers computing cut sparsifiers of weighted graphs of size O(nlog (n)/ε²). Our algorithm computes such a sparsifier in time O(m⋅min(α(n)log(m/n),log (n))), both for graphs with polynomially bounded and unbounded integer weights, where α(⋅) is the functional inverse of Ackermann’s function. This improves upon the state of the art by Benczúr and Karger (SICOMP 2015), which takes O(mlog² (n)) time. For unbounded weights, this directly gives the best known result for cut sparsification. Together with preprocessing by an algorithm of Fung et al. (SICOMP 2019), this also gives the best known result for polynomially-weighted graphs. Consequently, this implies the fastest approximate min-cut algorithm, both for graphs with polynomial and unbounded weights. In particular, we show that it is possible to adapt the state of the art algorithm of Fung et al. for unweighted graphs to weighted graphs, by letting the partial maximum spanning forest (MSF) packing take the place of the Nagamochi-Ibaraki (NI) forest packing. MSF packings have previously been used by Abraham at al. (FOCS 2016) in the dynamic setting, and are defined as follows: an M-partial MSF packing of G is a set ℱ = {F₁, … , F_M}, where F_i is a maximum spanning forest in G⧵ ⋃_{j = 1}^{i-1}F_j. Our method for computing (a sufficient estimation of) the MSF packing is the bottleneck in the running time of our sparsification algorithm.
Cite as

Sebastian Forster and Tijn de Vos. Faster Cut Sparsification of Weighted Graphs. In 49th International Colloquium on Automata, Languages, and Programming (ICALP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 229, pp. 61:1-61:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{forster_et_al:LIPIcs.ICALP.2022.61,
  author =	{Forster, Sebastian and de Vos, Tijn},
  title =	{{Faster Cut Sparsification of Weighted Graphs}},
  booktitle =	{49th International Colloquium on Automata, Languages, and Programming (ICALP 2022)},
  pages =	{61:1--61:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-235-8},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{229},
  editor =	{Boja\'{n}czyk, Miko{\l}aj and Merelli, Emanuela and Woodruff, David P.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2022.61},
  URN =		{urn:nbn:de:0030-drops-164029},
  doi =		{10.4230/LIPIcs.ICALP.2022.61},
  annote =	{Keywords: Cut Sparsification, Graph Algorithms}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2021.16
An Improved Random Shift Algorithm for Spanners and Low Diameter Decompositions

Authors: Sebastian Forster, Martin Grösbacher, and Tijn de Vos

Published in: LIPIcs, Volume 217, 25th International Conference on Principles of Distributed Systems (OPODIS 2021)

Abstract
Spanners have been shown to be a powerful tool in graph algorithms. Many spanner constructions use a certain type of clustering at their core, where each cluster has small diameter and there are relatively few spanner edges between clusters. In this paper, we provide a clustering algorithm that, given k ≥ 2, can be used to compute a spanner of stretch 2k-1 and expected size O(n^{1+1/k}) in k rounds in the CONGEST model. This improves upon the state of the art (by Elkin, and Neiman [TALG'19]) by making the bounds on both running time and stretch independent of the random choices of the algorithm, whereas they only hold with high probability in previous results. Spanners are used in certain synchronizers, thus our improvement directly carries over to such synchronizers. Furthermore, for keeping the total number of inter-cluster edges small in low diameter decompositions, our clustering algorithm provides the following guarantees. Given β ∈ (0,1], we compute a low diameter decomposition with diameter bound O((log n)/β) such that each edge e ∈ E is an inter-cluster edge with probability at most β⋅ w(e) in O((log n)/β) rounds in the CONGEST model. Again, this improves upon the state of the art (by Miller, Peng, and Xu [SPAA'13]) by making the bounds on both running time and diameter independent of the random choices of the algorithm, whereas they only hold with high probability in previous results.
Cite as

Sebastian Forster, Martin Grösbacher, and Tijn de Vos. An Improved Random Shift Algorithm for Spanners and Low Diameter Decompositions. In 25th International Conference on Principles of Distributed Systems (OPODIS 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 217, pp. 16:1-16:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{forster_et_al:LIPIcs.OPODIS.2021.16,
  author =	{Forster, Sebastian and Gr\"{o}sbacher, Martin and de Vos, Tijn},
  title =	{{An Improved Random Shift Algorithm for Spanners and Low Diameter Decompositions}},
  booktitle =	{25th International Conference on Principles of Distributed Systems (OPODIS 2021)},
  pages =	{16:1--16:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-219-8},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{217},
  editor =	{Bramas, Quentin and Gramoli, Vincent and Milani, Alessia},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2021.16},
  URN =		{urn:nbn:de:0030-drops-157914},
  doi =		{10.4230/LIPIcs.OPODIS.2021.16},
  annote =	{Keywords: Spanner, low diameter decomposition, synchronizer, distributed graph algorithms}
}
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