Small stretch (α,β)-spanners in the streaming model

  • Authors:
  • Giorgio Ausiello;Paolo G. Franciosa;Giuseppe F. Italiano

  • Affiliations:
  • Dipartimento di Informatica e Sistemistica, Sapienza Università di Roma, via Ariosto 25, 00185 Roma, Italy;Dipartimento di Statistica, Probabilità e Statistiche Applicate, Sapienza Università di Roma, piazzale Aldo Moro 5, I-00185 Roma, Italy;Dipartimento di Informatica, Sistemi e Produzione, Università di Roma Tor Vergata, via del Politecnico 1, 00133 Roma, Italy

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2009

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Abstract

We present algorithms for computing small stretch (@a,@b)-spanners in the streaming model. An (@a,@b)-spanner of a graph G is a subgraph S@?G such that for each pair of vertices the distance in S is at most @a times the distance in G plus @b. We assume that the graph is given as a stream of edges and vertices, and that only one pass over the data is allowed. Furthermore, the number of vertices and edges are not known in advance. We denote by m the current number of scanned edges and by n the current number of discovered vertices. In this model we show how to compute a (k,k-1)-spanner of an unweighted undirected graph, for k=2,3, in O(1) amortized processing time per edge/vertex. The computed (k,k-1)-spanners have O(n^1^+^1^/^k) edges and our algorithms use only O(n^1^+^1^/^k) words of memory space. In case only @Q(n) internal memory is available, the same spanners can be computed using O(n^1^+^1^/^k/B) external memory blocks, each of size B. Each edge/vertex is processed in O(1) amortized time, plus O(1/B) amortized block transfers.