# Tightness of measures

In mathematics, tightness is a concept in measure theory. The intuitive idea is that a given collection of measures does not "escape to infinity".

## Definitions

Let $(X,T)$ be a Hausdorff space, and let $\Sigma$ be a σ-algebra on $X$ that contains the topology $T$. (Thus, every open subset of $X$ is a measurable set and $\Sigma$ is at least as fine as the Borel σ-algebra on $X$.) Let $M$ be a collection of (possibly signed or complex) measures defined on $\Sigma$. The collection $M$ is called tight (or sometimes uniformly tight) if, for any $\varepsilon >0$, there is a compact subset $K_{\varepsilon }$ of $X$ such that, for all measures $\mu \in M$,
    $|\mu |(X\setminus K_{\varepsilon })<\varepsilon .$
where $|\mu |$ is the total variation measure of $\mu$. Very often, the measures in question are probability measures, so the last part can be written as
    $\mu (K_{\varepsilon })>1-\varepsilon .\,$
If a tight collection $M$ consists of a single measure $\mu$, then (depending upon the author) $\mu$ may either be said to be a tight measure or to be an inner regular measure.
If $Y$ is an $X$-valued random variable whose probability distribution on $X$ is a tight measure then $Y$ is said to be a separable random variable or a Radon random variable.
Another equivalent criterion of the tightness of a collection $M$ is sequentially weakly compact. We say the family $M$ of probability measures is sequentially weakly compact if for every sequence $\left\{\mu _{n}\right\}$ from the family, there is a subsequence of measures that converges weakly to some probability measure $\mu$. It can be shown that a family of measure is tight if and only if it is sequentially weakly compact.

## Examples
### Compact spaces

If $X$ is a metrizable compact space, then every collection of (possibly complex) measures on $X$ is tight. This is not necessarily so for non-metrisable compact spaces. If we take $[0,\omega _{1}]$ with its order topology, then there exists a measure $\mu$ on it that is not inner regular. Therefore, the singleton $\{\mu \}$ is not tight.

### Polish spaces

If $X$ is a Polish space, then every probability measure on $X$ is tight. Furthermore, by Prokhorov's theorem, a collection of probability measures on $X$ is tight if and only if it is precompact in the topology of weak convergence.

### A collection of point masses

Consider the real line $\mathbb {R}$ with its usual Borel topology. Let $\delta _{x}$ denote the Dirac measure, a unit mass at the point $x$ in $\mathbb {R}$. The collection $M_{1}:=\{\delta _{n}\mid n\in \mathbb {N} \}$ is not tight, since the compact subsets of $\mathbb {R}$ are precisely the closed and bounded subsets, and any such set, since it is bounded, has $\delta _{n}$-measure zero for large enough $n$. On the other hand, the collection $M_{2}:=\{\delta _{1/n}\mid n\in \mathbb {N} \}$ is tight: the compact interval $[0,1]$ will work as $K_{\varepsilon }$ for any $\varepsilon >0$. In general, a collection of Dirac delta measures on $\mathbb {R} ^{n}$ is tight if, and only if, the collection of their supports is bounded.

### A collection of Gaussian measures

Consider $n$-dimensional Euclidean space $\mathbb {R} ^{n}$ with its usual Borel topology and σ-algebra. Consider a collection of Gaussian measures $\Gamma =\{\gamma _{i}\mid i\in I\},$ where the measure $\gamma _{i}$ has expected value (mean) $m_{i}\in \mathbb {R} ^{n}$ and covariance matrix $C_{i}\in \mathbb {R} ^{n\times n}$. Then the collection $\Gamma$ is tight if, and only if, the collections $\{m_{i}\mid i\in I\}\subseteq \mathbb {R} ^{n}$ and $\{C_{i}\mid i\in I\}\subseteq \mathbb {R} ^{n\times n}$ are both bounded.

## Tightness and convergence

Tightness is often a necessary criterion for proving the weak convergence of a sequence of probability measures, especially when the measure space has infinite dimension. See
* Finite-dimensional distribution
* Prokhorov's theorem
* Lévy–Prokhorov metric
* Weak convergence of measures
* Tightness in classical Wiener space
* Tightness in Skorokhod space

## Tightness and stochastic ordering

A family of real-valued random variables $\{X_{i}\}_{i\in I}$ is tight if and only if there exists an almost surely finite random variable $X$ such that $|X_{i}|\leq _{\mathrm {st} }X$ for all $i\in I$, where $\leq _{\mathrm {st} }$ denotes the stochastic order defined by $A\leq _{\mathrm {st} }B$ if $E\phi (A)\leq E\phi (B)$ for all nondecreasing functions $\phi$.

## Exponential tightness

A strengthening of tightness is the concept of exponential tightness, which has applications in large deviations theory. A family of probability measures $(\mu _{\delta })_{\delta >0}$ on a Hausdorff topological space $X$ is said to be exponentially tight if, for any $\varepsilon >0$, there is a compact subset $K_{\varepsilon }$ of $X$ such that
    $\limsup _{\delta \downarrow 0}\delta \log \mu _{\delta }(X\setminus K_{\varepsilon })<-\varepsilon .$