Random element

Subsequently, the term "random element" was used chiefly with reference to "randomly chosen" element in some linear topological space, especially a Hilbert space or a Banach space. The exact definition of a random element in a Banach space , for example, is reminiscent of the definition of a random variable. Let be some probability space, let be a Banach space and let be the dual space of . A mapping from the space of elementary events into is called a random element if every continuous linear functional is actually a random variable, that is, an -measurable function.
Let be the smallest -algebra of subsets of with respect to which all continuous linear functionals are measurable. is a random element if and only if the complete pre-image of all sets in are -measurable. In the case when is separable, coincides with the -algebra of Borel subsets of .
The basic concepts of probability theory, such as the characteristic function, the mathematical expectation and the covariance, among other things, can be extended to random elements. A random element is called normal (Gaussian) if the probability distribution of every continuous linear functional is normal (cf. Normal distribution). The weak law of large numbers, the strong law of large numbers, the law of the iterated logarithm, the central limit theorem, and other assertions of probability theory can be extended to sequences of random elements. Whether these theorems in their classical form carry over to the case of Banach spaces depends on the geometry of the space. It is important to note that this is a two-way connection, in that the probabilistic properties often turn out to be in fact probabilistic-geometric: not only is their validity in a given Banach space determined by the geometric properties of the space, but conversely it determines these properties. E.g., for any sequence of independent identically-distributed random elements with values in , zero mathematical expectations and , the distribution of the normalized sum converges weakly to the distribution of a normal random element as if and only if is a so-called type-2 space (see ).