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Closed graph theorem (functional analysis)

## Summary

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.

## Explanation

Let ${\displaystyle T:X\to Y}$  be a linear operator between Banach spaces (or more generally Fréchet spaces). Then the continuity of ${\displaystyle T}$  means that ${\displaystyle Tx_{i}\to Tx}$  for each convergent sequence ${\displaystyle x_{i}\to x}$ . On the other hand, the closedness of the graph of ${\displaystyle T}$  means that for each convergent sequence ${\displaystyle x_{i}\to x}$  such that ${\displaystyle Tx_{i}\to y}$ , we have ${\displaystyle y=Tx}$ . Hence, the closed graph theorem says that in order to check the continuity of ${\displaystyle T}$ , one can show ${\displaystyle Tx_{i}\to Tx}$  under the additional assumption that ${\displaystyle Tx_{i}}$  is convergent.

In fact, for the graph of T to be closed, it is enough that if ${\displaystyle x_{i}\to 0,\,Tx_{i}\to y}$ , then ${\displaystyle y=0}$ . Indeed, assuming that condition holds, if ${\displaystyle (x_{i},Tx_{i})\to (x,y)}$ , then ${\displaystyle x_{i}-x\to 0}$  and ${\displaystyle T(x_{i}-x)\to y-Tx}$ . Thus, ${\displaystyle y=Tx}$ ; i.e., ${\displaystyle (x,y)}$  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.

## Statement

Theorem — [2] If ${\displaystyle T:X\to Y}$  is a linear operator between Banach spaces (or more generally Fréchet spaces), then the following are equivalent:

1. ${\displaystyle T}$  is continuous.
2. The graph of ${\displaystyle T}$  is closed in the product topology on ${\displaystyle X\times Y.}$

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 ${\displaystyle \Gamma _{T}}$  of T is closed. Then ${\displaystyle \Gamma _{T}\simeq \Gamma _{T^{-1}}}$  under ${\displaystyle (x,y)\mapsto (y,x)}$ . Hence, by the closed graph theorem, ${\displaystyle T^{-1}}$  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.

## Example

The Hausdorff–Young inequality says that the Fourier transformation ${\displaystyle {\widehat {\cdot }}:L^{p}(\mathbb {R} ^{n})\to L^{p'}(\mathbb {R} ^{n})}$  is a well-defined bounded operator with operator norm one when ${\displaystyle 1/p+1/p'=1}$ . 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 ${\displaystyle T:L^{p}\to Z}$  is a continuous linear operator for Z = the space of tempered distributions on ${\displaystyle \mathbb {R} ^{n}}$ . 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 ${\displaystyle L^{p}\times L^{p'}}$  and ${\displaystyle T:L^{p}\to L^{p'}}$  is defined but with unknown bounds.[clarification needed] Since ${\displaystyle T:L^{p}\to Z}$  is continuous, the graph of ${\displaystyle T:L^{p}\to L^{p'}}$  is closed in the distribution topology; thus in the norm topology. Finally, by the closed graph theorem, ${\displaystyle T:L^{p}\to L^{p'}}$  is a bounded operator.

## Generalization

### Complete metrizable codomain

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 ${\displaystyle X}$  to a Fréchet space ${\displaystyle Y}$  is continuous if and only if its graph is closed.

#### Between F-spaces

There are versions that does not require ${\displaystyle Y}$  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 ${\displaystyle T:X\to Y}$  is a linear map between two F-spaces, then the following are equivalent:

1. ${\displaystyle T}$  is continuous.
2. ${\displaystyle T}$  has a closed graph.
3. If ${\displaystyle x_{\bullet }=\left(x_{i}\right)_{i=1}^{\infty }\to x}$  in ${\displaystyle X}$  and if ${\displaystyle T\left(x_{\bullet }\right):=\left(T\left(x_{i}\right)\right)_{i=1}^{\infty }}$  converges in ${\displaystyle Y}$  to some ${\displaystyle y\in Y,}$  then ${\displaystyle y=T(x).}$ [6]
4. If ${\displaystyle x_{\bullet }=\left(x_{i}\right)_{i=1}^{\infty }\to 0}$  in ${\displaystyle X}$  and if ${\displaystyle T\left(x_{\bullet }\right)}$  converges in ${\displaystyle Y}$  to some ${\displaystyle y\in Y,}$  then ${\displaystyle y=0.}$

### Complete pseudometrizable codomain

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]

### Codomain not complete or (pseudo) metrizable

Theorem[8] — Suppose that ${\displaystyle T:X\to Y}$  is a linear map whose graph is closed. If ${\displaystyle X}$  is an inductive limit of Baire TVSs and ${\displaystyle Y}$  is a webbed space then ${\displaystyle T}$  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 ${\displaystyle X}$  and ${\displaystyle Y}$  are two topological vector spaces (they need not be Hausdorff or locally convex) with the following property:

If ${\displaystyle G}$  is any closed subspace of ${\displaystyle X\times Y}$  and ${\displaystyle u}$  is any continuous map of ${\displaystyle G}$  onto ${\displaystyle X,}$  then ${\displaystyle u}$  is an open mapping.

Under this condition, if ${\displaystyle T:X\to Y}$  is a linear map whose graph is closed then ${\displaystyle T}$  is continuous.

## Borel graph theorem

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 ${\displaystyle u:X\to Y}$  be linear map between two locally convex Hausdorff spaces ${\displaystyle X}$  and ${\displaystyle Y.}$  If ${\displaystyle X}$  is the inductive limit of an arbitrary family of Banach spaces, if ${\displaystyle Y}$  is a Souslin space, and if the graph of ${\displaystyle u}$  is a Borel set in ${\displaystyle X\times Y,}$  then ${\displaystyle u}$  is continuous.[10]

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

A topological space ${\displaystyle X}$  is called a ${\displaystyle K_{\sigma \delta }}$  if it is the countable intersection of countable unions of compact sets.

A Hausdorff topological space ${\displaystyle Y}$  is called K-analytic if it is the continuous image of a ${\displaystyle K_{\sigma \delta }}$  space (that is, if there is a ${\displaystyle K_{\sigma \delta }}$  space ${\displaystyle X}$  and a continuous map of ${\displaystyle X}$  onto ${\displaystyle Y}$ ).

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 ${\displaystyle u:X\to Y}$  be a linear map between two locally convex Hausdorff spaces ${\displaystyle X}$  and ${\displaystyle Y.}$  If ${\displaystyle X}$  is the inductive limit of an arbitrary family of Banach spaces, if ${\displaystyle Y}$  is a K-analytic space, and if the graph of ${\displaystyle u}$  is closed in ${\displaystyle X\times Y,}$  then ${\displaystyle u}$  is continuous.

If ${\displaystyle F:X\to Y}$  is closed linear operator from a Hausdorff locally convex TVS ${\displaystyle X}$  into a Hausdorff finite-dimensional TVS ${\displaystyle Y}$  then ${\displaystyle F}$  is continuous.[12]

## References

Notes

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

## Bibliography

• Adasch, Norbert; Ernst, Bruno; Keim, Dieter (1978). Topological Vector Spaces: The Theory Without Convexity Conditions. Lecture Notes in Mathematics. Vol. 639. Berlin New York: Springer-Verlag. ISBN 978-3-540-08662-8. OCLC 297140003.
• Banach, Stefan (1932). Théorie des Opérations Linéaires [Theory of Linear Operations] (PDF). Monografie Matematyczne (in French). Vol. 1. Warszawa: Subwencji Funduszu Kultury Narodowej. Zbl 0005.20901. Archived from the original (PDF) on 2014-01-11. Retrieved 2020-07-11.
• Berberian, Sterling K. (1974). Lectures in Functional Analysis and Operator Theory. Graduate Texts in Mathematics. Vol. 15. New York: Springer. ISBN 978-0-387-90081-0. OCLC 878109401.
• Bourbaki, Nicolas (1987) [1981]. Topological Vector Spaces: Chapters 1–5. Éléments de mathématique. Translated by Eggleston, H.G.; Madan, S. Berlin New York: Springer-Verlag. ISBN 3-540-13627-4. OCLC 17499190.
• Conway, John (1990). A course in functional analysis. Graduate Texts in Mathematics. Vol. 96 (2nd ed.). New York: Springer-Verlag. ISBN 978-0-387-97245-9. OCLC 21195908.
• Edwards, Robert E. (1995). Functional Analysis: Theory and Applications. New York: Dover Publications. ISBN 978-0-486-68143-6. OCLC 30593138.
• Dolecki, Szymon; Mynard, Frederic (2016). Convergence Foundations Of Topology. New Jersey: World Scientific Publishing Company. ISBN 978-981-4571-52-4. OCLC 945169917.
• Dubinsky, Ed (1979). The Structure of Nuclear Fréchet Spaces. Lecture Notes in Mathematics. Vol. 720. Berlin New York: Springer-Verlag. ISBN 978-3-540-09504-0. OCLC 5126156.
• Grothendieck, Alexander (1973). Topological Vector Spaces. Translated by Chaljub, Orlando. New York: Gordon and Breach Science Publishers. ISBN 978-0-677-30020-7. OCLC 886098.
• Husain, Taqdir; Khaleelulla, S. M. (1978). Barrelledness in Topological and Ordered Vector Spaces. Lecture Notes in Mathematics. Vol. 692. Berlin, New York, Heidelberg: Springer-Verlag. ISBN 978-3-540-09096-0. OCLC 4493665.
• Jarchow, Hans (1981). Locally convex spaces. Stuttgart: B.G. Teubner. ISBN 978-3-519-02224-4. OCLC 8210342.
• Köthe, Gottfried (1983) [1969]. Topological Vector Spaces I. Grundlehren der mathematischen Wissenschaften. Vol. 159. Translated by Garling, D.J.H. New York: Springer Science & Business Media. ISBN 978-3-642-64988-2. MR 0248498. OCLC 840293704.
• Kriegl, Andreas; Michor, Peter W. (1997). The Convenient Setting of Global Analysis (PDF). Mathematical Surveys and Monographs. Vol. 53. Providence, R.I: American Mathematical Society. ISBN 978-0-8218-0780-4. OCLC 37141279.
• Munkres, James R. (2000). Topology (Second ed.). Upper Saddle River, NJ: Prentice Hall, Inc. ISBN 978-0-13-181629-9. OCLC 42683260.
• Narici, Lawrence; Beckenstein, Edward (2011). Topological Vector Spaces. Pure and applied mathematics (Second ed.). Boca Raton, FL: CRC Press. ISBN 978-1584888666. OCLC 144216834.
• Robertson, Alex P.; Robertson, Wendy J. (1980). Topological Vector Spaces. Cambridge Tracts in Mathematics. Vol. 53. Cambridge England: Cambridge University Press. ISBN 978-0-521-29882-7. OCLC 589250.
• Rudin, Walter (1991). Functional Analysis. International Series in Pure and Applied Mathematics. Vol. 8 (Second ed.). New York, NY: McGraw-Hill Science/Engineering/Math. ISBN 978-0-07-054236-5. OCLC 21163277.
• Schaefer, Helmut H.; Wolff, Manfred P. (1999). Topological Vector Spaces. GTM. Vol. 8 (Second ed.). New York, NY: Springer New York Imprint Springer. ISBN 978-1-4612-7155-0. OCLC 840278135.
• Swartz, Charles (1992). An introduction to Functional Analysis. New York: M. Dekker. ISBN 978-0-8247-8643-4. OCLC 24909067.
• Tao, Terence, 245B, Notes 9: The Baire category theorem and its Banach space consequences
• Trèves, François (2006) [1967]. Topological Vector Spaces, Distributions and Kernels. Mineola, N.Y.: Dover Publications. ISBN 978-0-486-45352-1. OCLC 853623322.
• Vogt, Dietmar (2000), Lectures on Fréchet spaces (PDF)
• Wilansky, Albert (2013). Modern Methods in Topological Vector Spaces. Mineola, New York: Dover Publications, Inc. ISBN 978-0-486-49353-4. OCLC 849801114.
• "Proof of closed graph theorem". PlanetMath.