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In mathematics, particularly in functional analysis, the **closed graph theorem** is a result connecting the continuity of a linear operator to a topological property of their graph. Precisely, the theorem states that a linear operator between two Banach spaces is continuous if and only if the graph of the operator is closed (such an operator is called a closed linear operator; see also closed graph property).

One of important questions in functional analysis is the question of the continuity (or boundedness) of a given linear operator. The closed graph theorem gives one answer to that question.

Let be a linear operator between Banach spaces (or more generally Fréchet spaces). Then the continuity of means that for each convergent sequence . On the other hand, the closedness of the graph of means that for each convergent sequence such that , we have . Hence, the closed graph theorem says that in order to check the continuity of , one can show under the additional assumption that is convergent.

In fact, for the graph of *T* to be closed, it is enough that if , then . Indeed, assuming that condition holds, if , then and . Thus, ; i.e., is in the graph of *T*.

Note, to check the closedness of a graph, it’s not even necessary to use the norm topology: if the graph of *T* is closed in some topology coarser than the norm topology, then it is closed in the norm topology.^{[1]} In practice, this works like this: *T* is some operator on some function space. One shows *T* is continuous with respect to the distribution topology; thus, the graph is closed in that topology, which implies closedness in the norm topology and then *T* is a bounded by the closed graph theorem (when the theorem applies). See § Example for an explicit example.

**Theorem** — ^{[2]}
If is a linear operator between Banach spaces (or more generally Fréchet spaces), then the following are equivalent:

- is continuous.
- The graph of is closed in the product topology on

The usual proof of the closed graph theorem employs the open mapping theorem. It simply uses a general recipe of obtaining the closed graph theorem from the open mapping theorem; see closed graph theorem § Relation to the open mapping theorem (this deduction is formal and does not use linearity; the linearity is needed to appeal to the open mapping theorem which relies on the linearity.)

In fact, the open mapping theorem can in turn be deduced from the closed graph theorem as follows. As noted in Open mapping theorem (functional analysis) § Statement and proof, it is enough to prove the open mapping theorem for a continuous linear operator that is bijective (not just surjective). Let *T* be such an operator. Then by continuity, the graph of *T* is closed. Then under . Hence, by the closed graph theorem, is continuous; i.e., *T* is an open mapping.

Since the closed graph theorem is equivalent to the open mapping theorem, one knows that the theorem fails without the completeness assumption. But more concretely, an operator with closed graph that is not bounded (see unbounded operator) exists and thus serves as a counterexample.

The Hausdorff–Young inequality says that the Fourier transformation is a well-defined bounded operator with operator norm one when . This result is usually proved using the Riesz–Thorin interpolation theorem and is highly nontrivial. The closed graph theorem can be used to prove a soft version of this result; i.e., the Fourier transformation is a bounded operator with the unknown operator norm.^{[3]}

Here is how the argument would go. Let *T* denote the Fourier transformation. First we show is a continuous linear operator for *Z* = the space of tempered distributions on . Second, we note that *T* maps the space of Schwarz functions to itself (in short, because smoothness and rapid decay transform to rapid decay and smoothness, respectively). This implies that the graph of *T* is contained in and is defined but with unknown bounds.^{[clarification needed]} Since is continuous, the graph of is closed in the distribution topology; thus in the norm topology. Finally, by the closed graph theorem, is a bounded operator.

The closed graph theorem can be generalized from Banach spaces to more abstract topological vector spaces in the following ways.

**Theorem** — A linear operator from a barrelled space to a Fréchet space is continuous if and only if its graph is closed.

There are versions that does not require to be locally convex.

**Theorem** — A linear map between two F-spaces is continuous if and only if its graph is closed.^{[4]}^{[5]}

This theorem is restated and extend it with some conditions that can be used to determine if a graph is closed:

**Theorem** — If is a linear map between two F-spaces, then the following are equivalent:

- is continuous.
- has a closed graph.
- If in and if converges in to some then
^{[6]} - If in and if converges in to some then

Every metrizable topological space is pseudometrizable. A pseudometrizable space is metrizable if and only if it is Hausdorff.

**Closed Graph Theorem ^{[7]}** — Also, a closed linear map from a locally convex ultrabarrelled space into a complete pseudometrizable TVS is continuous.

**Closed Graph Theorem** — A closed and bounded linear map from a locally convex infrabarreled space into a complete pseudometrizable locally convex space is continuous.^{[7]}

**Theorem ^{[8]}** — Suppose that is a linear map whose graph is closed.
If is an inductive limit of Baire TVSs and is a webbed space then is continuous.

**Closed Graph Theorem ^{[7]}** — A closed surjective linear map from a complete pseudometrizable TVS onto a locally convex ultrabarrelled space is continuous.

An even more general version of the closed graph theorem is

**Theorem ^{[9]}** — Suppose that and are two topological vector spaces (they need not be Hausdorff or locally convex) with the following property:

- If is any closed subspace of and is any continuous map of onto then is an open mapping.

Under this condition, if is a linear map whose graph is closed then is continuous.

The Borel graph theorem, proved by L. Schwartz, shows that the closed graph theorem is valid for linear maps defined on and valued in most spaces encountered in analysis.^{[10]}
Recall that a topological space is called a Polish space if it is a separable complete metrizable space and that a Souslin space is the continuous image of a Polish space. The weak dual of a separable Fréchet space and the strong dual of a separable Fréchet-Montel space are Souslin spaces. Also, the space of distributions and all Lp-spaces over open subsets of Euclidean space as well as many other spaces that occur in analysis are Souslin spaces.
The Borel graph theorem states:

**Borel Graph Theorem** — Let be linear map between two locally convex Hausdorff spaces and If is the inductive limit of an arbitrary family of Banach spaces, if is a Souslin space, and if the graph of is a Borel set in then is continuous.^{[10]}

An improvement upon this theorem, proved by A. Martineau, uses K-analytic spaces.

A topological space is called a if it is the countable intersection of countable unions of compact sets.

A Hausdorff topological space is called **K-analytic** if it is the continuous image of a space (that is, if there is a space and a continuous map of onto ).

Every compact set is K-analytic so that there are non-separable K-analytic spaces. Also, every Polish, Souslin, and reflexive Fréchet space is K-analytic as is the weak dual of a Frechet space. The generalized Borel graph theorem states:

**Generalized Borel Graph Theorem ^{[11]}** — Let be a linear map between two locally convex Hausdorff spaces and If is the inductive limit of an arbitrary family of Banach spaces, if is a K-analytic space, and if the graph of is closed in then is continuous.

If is closed linear operator from a Hausdorff locally convex TVS into a Hausdorff finite-dimensional TVS then is continuous.^{[12]}

- Almost open linear map – Map that satisfies a condition similar to that of being an open map.
- Barrelled space – Type of topological vector space
- Closed graph – Graph of a map closed in the product space
- Closed linear operator
- Densely defined operator – Function that is defined almost everywhere (mathematics)
- Discontinuous linear map
- Kakutani fixed-point theorem – Fixed-point theorem for set-valued functions
- Open mapping theorem (functional analysis) – Condition for a linear operator to be open
- Ursescu theorem – Generalization of closed graph, open mapping, and uniform boundedness theorem
- Webbed space – Space where open mapping and closed graph theorems hold

**Notes**

**^**Theorem 4 of Tao. NB: The Hausdorffness there is put to ensure the graph of a continuous map is closed.**^**Vogt 2000, Theorem 1.8.**^**Tao, Example 3**^**Schaefer & Wolff 1999, p. 78.**^**Trèves (2006), p. 173**^**Rudin 1991, pp. 50–52.- ^
^{a}^{b}^{c}Narici & Beckenstein 2011, pp. 474–476. **^**Narici & Beckenstein 2011, p. 479-483.**^**Trèves 2006, p. 169.- ^
^{a}^{b}Trèves 2006, p. 549. **^**Trèves 2006, pp. 557–558.**^**Narici & Beckenstein 2011, p. 476.

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