Gaussian measure


In mathematics, Gaussian measure is a Borel measure on finite-dimensional Euclidean space Rn, closely related to the normal distribution in statistics. There is also a generalization to infinite-dimensional spaces. Gaussian measures are named after the German mathematician Carl Friedrich Gauss. One reason why Gaussian measures are so ubiquitous in probability theory is the central limit theorem. Loosely speaking, it states that if a random variable X is obtained by summing a large number N of independent random variables of order 1, then X is of order and its law is approximately Gaussian.

Definitions

Let n ∈ N and let B0 denote the completion of the Borel σ-algebra on Rn. Let λn : B0 → denote the usual n-dimensional Lebesgue measure. Then the standard Gaussian measure γn : B0 → is defined by
for any measurable set A ∈ B0. In terms of the Radon–Nikodym derivative,
More generally, the Gaussian measure with mean μ ∈ Rn and variance σ2 > 0 is given by
Gaussian measures with mean μ = 0 are known as centred Gaussian measures.
The Dirac measure δμ is the weak limit of as σ → 0, and is considered to be a degenerate Gaussian measure; in contrast, Gaussian measures with finite, non-zero variance are called non-degenerate Gaussian measures.

Properties of Gaussian measure

The standard Gaussian measure γn on Rn
so Gaussian measure is a Radon measure;
It can be shown that there is no analogue of Lebesgue measure on an infinite-dimensional vector space. Even so, it is possible to define Gaussian measures on infinite-dimensional spaces, the main example being the abstract Wiener space construction. A Borel measure γ on a separable Banach space E is said to be a non-degenerate Gaussian measure if, for every linear functional L ∈ E∗ except L = 0, the push-forward measure L∗ is a non-degenerate Gaussian measure on R in the sense defined above.
For example, classical Wiener measure on the space of continuous paths is a Gaussian measure.