Abstract
In this paper we study the online variant of two well-known Steiner tree problems. In the online setting, the input consists of a sequence of terminals; upon arrival of a terminal, the online algorithm must irrevocably buy a subset of edges and vertices of the graph so as to guarantee the connectivity of the currently revealed part of the input. More precisely, we first study the online node-weighted Steiner tree problem, in which both edges and vertices are weighted, and the objective is to minimize the total cost of edges and vertices in the solution. We then address the online priority Steiner tree problem, in which each edge and each request are associated with a priority value, which corresponds to their bandwidth support and requirement, respectively. Both problems have applications in the domain of multicast network communications and have been studied from the point of view of approximation algorithms. Motivated by the observation that competitive analysis gives very pessimistic and unsatisfactory results when the only relevant parameter is the number of terminals, we introduce an approach based on parameterized analysis of online algorithms. In particular, we base the analysis on additional parameters that help reveal the true complexity of the underlying problem, and allow a much finer classification of online algorithms based on their performance. More specifically, for the online node-weighted Steiner tree problem, we show a tight bound of Θ(max{min{α,k},log k}) on the competitive ratio, where α is the ratio of the maximum node weight to the minimum node weight and k is the number of terminals. For the online priority Steiner tree problem, we show corresponding tight bounds of \({\Theta }(b\log \frac {k}{b})\), when k > b and Θ(k), when k ≤ b, where b is the number of priority levels and k is the number of terminals. Our main results apply to both deterministic and randomized algorithms, as well as to generalized versions of the problems (i.e., to Steiner forest variants).
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Notes
Throughout the paper we omit floors and ceilings since they do not affect the asymptotic behavior of the algorithms.
We say that a connection path for a terminal crosses an s-set (or a corresponding u-vertex) if it contains (vertical) edges in a column that is crossed by the s-set).
Without loss of generality we once again omit flours and ceilings, and assume all fraction quantities (as well as γ) are integral.
The weight w(e) of an edge should not be confused with its cost c(e).
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This paper is an extended combined version of two conference papers: The Node-Weighted Steiner Problem in Graphs of Restricted Node Weights, Proceedings of the 10th Scandinavian Workshop on Algorithm Theory, pp 208–219, 2006 and Online Priority Steiner Tree Problems, Proceedings of the 11th Symposium on Algorithms and Data Structures, pp 27–48, 2009.
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Angelopoulos, S. Parameterized Analysis of the Online Priority and Node-Weighted Steiner Tree Problems. Theory Comput Syst 63, 1413–1447 (2019). https://doi.org/10.1007/s00224-019-09922-2
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DOI: https://doi.org/10.1007/s00224-019-09922-2