Centred family of sets

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A family in which the intersection of any finite set of elements is non-empty. For example, the countable family of subsets of the series of natural numbers of the form is centred; any family in which the intersection of all elements is not empty is centred. Every finite centred family of sets has this last-named property.

Infinite centred families of sets were first used in general topology to characterize compact spaces. Centred families of closed sets in a topological space are used for the construction of its compactification and its absolute.

The concept of a centred system of sets can be generalized as follows. Let be an infinite cardinal number. Then an -centred family of sets is defined as a family for which the intersection of any set of elements of cardinality less than is not empty. Such families are used to characterize -compact spaces in abstract measure theory.


[1] J.L. Kelley, "General topology" , Springer (1975)
[2] L. Gillman, M. Jerison, "Rings of continuous functions" , v. Nostrand-Reinhold (1960)


A centred family of sets is also called a filtered family of sets or simply a filter.

How to Cite This Entry:
Centred family of sets. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by B.A. Efimov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article