Draft:Semitopology

Wikipedia

  • Comment: Sources 1 and 2 are not intellectually independent of each other. Source 3 isn't significant coverage of semitopology. Anerdw (talk) 05:27, 19 October 2025 (UTC)
  • Comment: Certainly a notable topic, but most of the article is unsourced. GGOTCC 03:02, 18 October 2025 (UTC)



Semitopology generalises general topology by removing the condition that the intersection of two open sets must be open.[1]

Definition

A semitopology is a pair (X,Opens) of a set X and a subset Opens ⊆ Powerset(X) such that Opens contains the empty set, X, and is closed under arbitrary sets unions.[1]:Definition 1.1.2

Motivation

Open sets of semitopologies abstract the notion of quorum from distributed computing.

An open set is interpreted as an actionable coalition, this being a set of participants in a distributed system with the power to collaborate to act to advance their local state (i.e. to take a step in some distributed protocol).

Using this interpretation, aspects of distributed algorithms can be framed using topological language. Notably:

  1. The notion of topological continuity can be identified with consensus.[1]:Lemma 2.2.4
  2. The characteristic quorum intersection property that any two quorums intersect,[2]:2.7.3 Quorums can be identified with the negation of the T2 Hausdorff separation property that any two distinct points have non-intersecting open neighbourhoods.[1]:Remark 3.6.10

Examples

  1. Every topology is trivially a semitopology.
  2. Let X be a finite set with n elements. Write MoreThan(k,X) for the set of subsets of X that contain more than k elements. Then (X,MoreThan(k,X)) is a semitopology. This semitopology is not in general a topology.

References

  1. 1 2 3 4 Gabbay, Murdoch (20 January 2025). "Semitopology: a topological approach to decentralized collaborative action". Journal of Logic and Computation. 35 (5) exae050. doi:10.1093/logcom/exae050.
  2. Cachin, Christian; Guerraoui, Rachid; Rodrigues, Luı́s (2011). Introduction to Reliable and Secure Distributed Systems (2 ed.). Springer. ISBN 978-3-642-15259-7.