Lévy–Prokhorov metric
In mathematics, the Lévy–Prokhorov metric (sometimes known just as the Prokhorov metric) is a metric (i.e., a definition of distance) on the collection of probability measures on a given metric space. It is named after the French mathematician Paul Lévy and the Soviet mathematician Yuri Vasilyevich Prokhorov; Prokhorov introduced it in 1956 as a generalization of the earlier Lévy metric.
Contents
Definition
Let be a metric space with its Borel sigma algebra
. Let
denote the collection of all probability measures on the measurable space
.
For a subset , define the ε-neighborhood of
by
where is the open ball of radius
centered at
.
The Lévy–Prokhorov metric is defined by setting the distance between two probability measures
and
to be
For probability measures clearly .
Some authors omit one of the two inequalities or choose only open or closed ; either inequality implies the other, and
, but restricting to open sets may change the metric so defined (if
is not Polish).
Properties
- If
is separable, convergence of measures in the Lévy–Prokhorov metric is equivalent to weak convergence of measures. Thus,
is a metrization of the topology of weak convergence on
.
- The metric space
is separable if and only if
is separable.
- If
is complete then
is complete. If all the measures in
have separable support, then the converse implication also holds: if
is complete then
is complete.
- If
is separable and complete, a subset
is relatively compact if and only if its
-closure is
-compact.
See also
- Lévy metric
- Prokhorov's theorem
- Tightness of measures
- weak convergence of measures
- Wasserstein metric
References
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