Closed-graph theorem

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Let and be complete metric linear spaces with translation-invariant metrics, i.e. , (similarly for ), and let be a linear operator from to . If the graph of this operator is a closed subset of the Cartesian product , then is continuous. The closed-graph theorem has various generalizations; for example: a linear mapping with closed graph from a separable barrelled space into a perfectly-complete space is continuous. Closely related theorems are the open-mapping theorem and Banach's homeomorphism theorem.


[1] W. Rudin, "Functional analysis" , McGraw-Hill (1979)
[2] A.P. Robertson, W.S. Robertson, "Topological vector spaces" , Cambridge Univ. Press (1964)


Cf. also Open-mapping theorem (also for the Banach homeomorphism theorem).

How to Cite This Entry:
Closed-graph theorem. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by V.I. Sobolev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article