Uniform Integrability

In mathematics, uniform integrability is an important concept in real analysis, functional analysis and measure theory, and plays a vital role in the theory of martingales.

Measure-theoretic definition

Uniform integrability is an extension to the notion of a family of functions being dominated in $L_1$ which is central in dominated convergence.
Several textbooks on real analysis and measure theory use the following definition:

Definition A: Let $(X,\mathfrak, \mu)$ be a positive measure space. A set $\Phi\subset L^1(\mu)$ is called uniformly integrable if $\sup_\, f\, _<\infty$, and to each $\varepsilon>0$ there corresponds a $\delta>0$ such that

: $\int_E , f, \, d\mu < \varepsilon$

whenever $f \in \Phi$ and $\mu(E)<\delta.$

Definition A is rather restrictive for infinite measure spaces. A more general definition of uniform integrability that works well in general measures spaces was introduced by G. A. Hunt.

Definition H: Let $(X,\mathfrak,\mu)$ be a positive measure space. A set $\Phi\subset L^1(\mu)$ is called uniformly integrable if and only if

:$\inf_\sup_\int_, f, \, d\mu=0$

where $L^1_+(\mu)=\$.

Since Hunt's definition is equivalent to Definition A when the underlying measure space is finite (see Theorem 2 below), Definition H is widely adopted in Mathematics.

The following result provides another equivalent notion to Hunt's. This equivalency is sometimes given as definition for uniform integrability.

Theorem 1: If $(X,\mathfrak,\mu)$ is a (positive) finite measure space, then a set $\Phi\subset L^1(\mu)$ is uniformly integrable if and only if

:$\inf_\sup_\int (, f, - g)^+ \, d\mu=0$

If in addition $\mu(X)<\infty$, then uniform integrability is equivalent to either of the following conditions

1. $\inf_\sup_\int(, f, -a)_+\,d\mu =0$.

2. $\inf_\sup_\int_, f, \,d\mu=0$

When the underlying space $(X,\mathfrak,\mu)$ is $\sigma$-finite, Hunt's definition is equivalent to the following:

Theorem 2: Let $(X,\mathfrak,\mu)$ be a $\sigma$-finite measure space, and $h\in L^1(\mu)$ be such that $h>0$ almost everywhere. A set $\Phi\subset L^1(\mu)$ is uniformly integrable if and only if $\sup_\, f\, _<\infty$, and for any $\varepsilon>0$, there exits $\delta>0$ such that

:$\sup_\int_A, f, \, d\mu <\varepsilon$

whenever $\int_A h\,d\mu <\delta$.

A consequence of Theorems 1 and 2 is that equivalence of Definitions A and H for finite measures follows. Indeed, the statement in Definition A is obtained by taking $h\equiv1$ in Theorem 2.

Probability definition

In the theory of probability, Definition A or the statement of Theorem 1 are often presented as definitions of uniform integrability using the notation expectation of random variables., that is,

1. A class $\mathcal$ of random variables is called uniformly integrable if:

* There exists a finite $M$ such that, for every $X$ in $\mathcal$, $\operatorname E(, X, )\leq M$ and
* For every $\varepsilon > 0$ there exists $\delta > 0$ such that, for every measurable $A$ such that $P(A)\leq \delta$ and every $X$ in $\mathcal$, $\operatorname E(, X, I_A)\leq\varepsilon$.

or alternatively

2. A class $\mathcal$ of random variables is called uniformly integrable (UI) if for every $\varepsilon > 0$ there exists K\in X, I_)\le\varepsilon\ \text X \in \mathcal, where $I_$ is the indicator function $I_ = \begin 1 &\text , X, \geq K, \\ 0 &\text , X, < K. \end$.

Tightness and uniform integrability

Another concept associated with uniform integrability is that of tightness. In this article tightness is taken in a more general setting.

Definition: Suppose measurable space $(X,\mathfrak,\mu)$ is a measure space. Let $\mathcal\subset\mathfrak$ be a collection of sets of finite measure. A family $\Phi\subset L_1(\mu)$ is tight with respect to $\mathcal$ if
:$\inf_\sup_\int_, f, \,\mu=0$
A tight family with respect to $\Phi=\mathfrak\cap L_1(\,u)$ is just said to be tight.

When the measure space $(X,\mathfrak,\mu)$ is a metric space equipped with the Borel $\sigma$ algebra, $\mu$ is a Regular measure, regular measure, and $\mathcal$ is the collection of all compact subsets of $X$, the notion of $\mathcal$-tightness discussed above coincides with the well known concept of tightness used in the analysis of regular measures in metric spaces

For $\sigma$-finite measure spaces, it can be shown that if a family $\Phi\subset L_1(\mu)$ is uniformly integrable, then $\Phi$ is tight. This is capture by the following result which is often used as definition of uniform integrabiliy in the Analysis literature:

Theorem 3: Suppose $(X,\mathfrak,\mu)$ is a $\sigma$ finite measure space. A family $\Phi\subset L_1(\mu)$ is uniformly integrable if and only if

# $\sup_\, f\, _1<\infty$.
# $\inf_\sup_\int_, f, \,d\mu=0$
# $\Phi$ is tight.

When $\mu(X)<\infty$, condition 3 is redundant (see Theorem 1 above).

Uniform absolute continuity

There is another notion of uniformity, slightly different than uniform integrability, which also has many applications in probability and measure theory, and which does not require random variables to have a finite integral

Definition: Suppose $(\Omega,\mathcal,P)$ is a probability space. A class $\mathcal$ of random variables is uniformly absolutely continuous with respect to $P$ if for any $\varepsilon>0$, there is $\delta>0$ such that
E absolute continuity