The Complexity of Optimal Design of Temporally Connected Graphs

Eleni C. Akrida, Leszek Gąsieniec, George B. Mertzios, Paul G. Spirakis

Research output: Contribution to journalArticlepeer-review

31 Scopus citations


We study the design of small cost temporally connected graphs, under various constraints. We mainly consider undirected graphs of n vertices, where each edge has an associated set of discrete availability instances (labels). A journey from vertex u to vertex v is a path from u to v where successive path edges have strictly increasing labels. A graph is temporally connected iff there is a (u, v)-journey for any pair of vertices u, v, u ≠ v. We first give a simple polynomial-time algorithm to check whether a given temporal graph is temporally connected. We then consider the case in which a designer of temporal graphs can freely choose availability instances for all edges and aims for temporal connectivity with very small cost; the cost is the total number of availability instances used. We achieve this via a simple polynomial-time procedure which derives designs of cost linear in n. We also show that the above procedure is (almost) optimal when the underlying graph is a tree, by proving a lower bound on the cost for any tree. However, there are pragmatic cases where one is not free to design a temporally connected graph anew, but is instead given a temporal graph design with the claim that it is temporally connected, and wishes to make it more cost-efficient by removing labels without destroying temporal connectivity (redundant labels). Our main technical result is that computing the maximum number of redundant labels is APX-hard, i.e., there is no PTAS unless P = NP. On the positive side, we show that in dense graphs with random edge availabilities, there is asymptotically almost surely a very large number of redundant labels. A temporal design may, however, be minimal, i.e., no redundant labels exist. We show the existence of minimal temporal designs with at least nlogn labels.

Original languageEnglish (US)
Pages (from-to)907-944
Number of pages38
JournalTheory of Computing Systems
Issue number3
StatePublished - Oct 1 2017
Externally publishedYes


  • APX-hard
  • Minimal graph
  • Network design
  • Random input
  • Temporal graphs
  • Temporally connected

ASJC Scopus subject areas

  • Theoretical Computer Science
  • Computational Theory and Mathematics


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