Arzelà–Ascoli Theorem

The Arzelà–Ascoli theorem is a fundamental result of mathematical analysis giving necessary and sufficient conditions to decide whether every sequence of a given family of real-valued continuous functions defined on a closed and bounded interval has a uniformly convergent subsequence. The main condition is the equicontinuity of the family of functions. The theorem is the basis of many proofs in mathematics, including that of the Peano existence theorem in the theory of ordinary differential equations, Montel's theorem in complex analysis, and the Peter–Weyl theorem in harmonic analysis and various results concerning compactness of integral operators.

The notion of equicontinuity was introduced in the late 19th century by the Italian mathematicians Cesare Arzelà and Giulio Ascoli. A weak form of the theorem was proven by , who established the sufficient condition for compactness, and by , who established the necessary condition and gave the first clear presentation of the result. A further generalization of the theorem was proven by , to sets of real-valued continuous functions with domain a compact metric space . Modern formulations of the theorem allow for the domain to be compact Hausdorff and for the range to be an arbitrary metric space. More general formulations of the theorem exist that give necessary and sufficient conditions for a family of functions from a compactly generated Hausdorff space into a uniform space to be compact in the compact-open topology; see .

Statement and first consequences

By definition, a sequence $\_$ of continuous functions on an interval is ''uniformly bounded'' if there is a number such that

:$\left, f_n(x)\ \le M$

for every function belonging to the sequence, and every . (Here, must be independent of and .)

The sequence is said to be ''uniformly equicontinuous'' if, for every , there exists a such that

:$\left, f_n(x)-f_n(y)\ < \varepsilon$

whenever for all functions in the sequence. (Here, may depend on , but not , or .)

One version of the theorem can be stated as follows:

:Consider a sequence of real-valued continuous functions defined on a closed and bounded interval of the real line. If this sequence is uniformly bounded and uniformly equicontinuous, then there exists a subsequence that converges uniformly.
:The converse is also true, in the sense that if every subsequence of itself has a uniformly convergent subsequence, then is uniformly bounded and equicontinuous.

Immediate examples

Differentiable functions

The hypotheses of the theorem are satisfied by a uniformly bounded sequence of differentiable functions with uniformly bounded derivatives. Indeed, uniform boundedness of the derivatives implies by the mean value theorem that for all and ,

:$\left, f_n(x) - f_n(y)\ \le K , x-y, ,$

where ''K'' is the supremum of the derivatives of functions in the sequence and is independent of . So, given , let to verify the definition of equicontinuity of the sequence. This proves the following corollary:

* Let be a uniformly bounded sequence of real-valued differentiable functions on such that the derivatives are uniformly bounded. Then there exists a subsequence that converges uniformly on .

If, in addition, the sequence of second derivatives is also uniformly bounded, then the derivatives also converge uniformly (up to a subsequence), and so on. Another generalization holds for continuously differentiable functions. Suppose that the functions are continuously differentiable with derivatives . Suppose that ''f_n''′ are uniformly equicontinuous and uniformly bounded, and that the sequence is pointwise bounded (or just bounded at a single point). Then there is a subsequence of the converging uniformly to a continuously differentiable function.

The diagonalization argument can also be used to show that a family of infinitely differentiable functions, whose derivatives of each order are uniformly bounded, has a uniformly convergent subsequence, all of whose derivatives are also uniformly convergent. This is particularly important in the theory of distributions.

Lipschitz and Hölder continuous functions

The argument given above proves slightly more, specifically

* If is a uniformly bounded sequence of real valued functions on such that each ''f'' is Lipschitz continuous with the same Lipschitz constant :

::$\left, f_n(x) - f_n(y)\ \le K, x-y,$

:for all and all , then there is a subsequence that converges uniformly on .

The limit function is also Lipschitz continuous with the same value for the Lipschitz constant. A slight refinement is

* A set of functions on that is uniformly bounded and satisfies a Hölder condition of order , , with a fixed constant ,

::\left, f(x) - f(y)\ \le M \, , x - y, ^\alpha, \qquad x, y \in , b/math>

:is relatively compact in . In particular, the unit ball of the Hölder space is compact in .

This holds more generally for scalar functions on a compact metric space satisfying a Hölder condition with respect to the metric on .

Generalizations

Euclidean spaces

The Arzelà–Ascoli theorem holds, more generally, if the functions take values in -dimensional Euclidean space , and the proof is very simple: just apply the -valued version of the Arzelà–Ascoli theorem times to extract a subsequence that converges uniformly in the first coordinate, then a sub-subsequence that converges uniformly in the first two coordinates, and so on. The above examples generalize easily to the case of functions with values in Euclidean space.

Compact metric spaces and compact Hausdorff spaces

The definitions of boundedness and equicontinuity can be generalized to the setting of arbitrary compact metric spaces and, more generally still, compact Hausdorff spaces. Let ''X'' be a compact Hausdorff space, and let ''C''(''X'') be the space of real-valued continuous functions on ''X''. A subset is said to be ''equicontinuous'' if for every ''x'' ∈ ''X'' and every , ''x'' has a neighborhood ''U_x'' such that

:$\forall y \in U_x, \forall f \in \mathbf : \qquad , f(y) - f(x), < \varepsilon.$

A set is said to be ''pointwise bounded'' if for every ''x'' ∈ ''X'',

:$\sup \ < \infty.$

A version of the Theorem holds also in the space ''C''(''X'') of real-valued continuous functions on a compact Hausdorff space ''X'' :

:Let ''X'' be a compact Hausdorff space. Then a subset F of ''C''(''X'') is relatively compact in the topology induced by the uniform norm if and only if it is equicontinuous and pointwise bounded.

The Arzelà–Ascoli theorem is thus a fundamental result in the study of the algebra of continuous functions on a compact Hausdorff space.

Various generalizations of the above quoted result are possible. For instance, the functions can assume values in a metric space or (Hausdorff) topological vector space with only minimal changes to the statement (see, for instance, , ):

:Let ''X'' be a compact Hausdorff space and ''Y'' a metric space. Then is compact in the compact-open topology if and only if it is equicontinuous, pointwise relatively compact and closed.

Here pointwise relatively compact means that for each ''x'' ∈ ''X'', the set is relatively compact in ''Y''.

In the case that ''Y'' is complete, the proof given above can be generalized in a way that does not rely on the separability of the domain. On a compact Hausdorff space ''X'', for instance, the equicontinuity is used to extract, for each ε = 1/''n'', a finite open covering of ''X'' such that the oscillation of any function in the family is less than ε on each open set in the cover. The role of the rationals can then be played by a set of points drawn from each open set in each of the countably many covers obtained in this way, and the main part of the proof proceeds exactly as above. A similar argument is used as a part of the proof for the general version which does not assume completeness of ''Y''.

Functions on non-compact spaces

The Arzela-Ascoli theorem generalises to functions $X \rightarrow Y$ where $X$ is not compact. Particularly important are cases where $X$ is a topological vector space. Recall that if $X$
is a topological space and $Y$ is a uniform space (such as any metric space or any topological group, metrisable or not), there is the topology of compact convergence on the set $\mathfrak(X,Y)$ of functions $X \rightarrow Y$; it is set up so that a sequence (or more generally a
filter or net) of functions converges if and only if it converges ''uniformly'' on each compact subset of $X$. Let $\mathcal_c(X,Y)$ be the subspace of
$\mathfrak(X,Y)$ consisting of continuous functions, equipped with the topology of compact convergence.
Then one form of the Arzèla-Ascoli theorem is the following:

:Let $X$ be a topological space, $Y$ a Hausdorff uniform space and $H\subset\mathcal_c(X,Y)$ an equicontinuous set of continuous functions such that $H(x)$ is relatively compact in $Y$ for each $x\in X$. Then $H$ is relatively compact in $H\subset\mathcal_c(X,Y)$.

This theorem immediately gives the more specialised statements above in cases where $X$ is compact
and the uniform structure of $Y$ is given by a metric. There are a few other variants in terms of
the topology of precompact convergence or other related topologies on
$\mathfrak(X,Y)$. It is also possible to extend the statement to functions that are only continuous when restricted to the sets of a covering of $X$ by compact subsets. For details one can consult Bourbaki (1998), Chapter X, § 2, nr 5.

Non-continuous functions

Solutions of numerical schemes for parabolic equations are usually piecewise constant, and therefore not continuous, in time. As their jumps nevertheless tend to become small as the time step goes to $0$, it is possible to establish uniform-in-time convergence properties using a generalisation to non-continuous functions of the classical Arzelà–Ascoli theorem (see e.g. ).

Denote by $S(X,Y)$ the space of functions from $X$ to $Y$ endowed with the uniform metric

:$d_S(v,w)=\sup_d_Y(v(t),w(t)).$

Then we have the following:

:Let $X$ be a compact metric space and $Y$ a complete metric space. Let $\_$ be a sequence in $S(X,Y)$ such that there exists a function \omega:X\times X\to ,\infty/math> and a sequence \_\subset oscillation_

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