Kolmogorov's normability criterion
In mathematics, Kolmogorov's normability criterion is a theorem that provides a necessary and sufficient condition for a topological vector space to be normable, i.e. for the existence of a norm on the space that generates the given topology. The normability criterion can be seen as a result in same vein as the Nagata–Smirnov metrization theorem, which gives a necessary and sufficient condition for a topological space to be metrizable. The result was proved by the Russian mathematician Andrey Nikolayevich Kolmogorov in 1934.[1] [2] [3]
Statement of the theorem
It may be helpful to first recall the following terms:
- A topological vector space is a vector space equipped with a topology such that the vector space operations of scalar multiplication and vector addition are continuous.
- A topological topological vector space is called normable if there is a norm on such that the open balls of the norm generate the given topology . (Note well that a given normable topological vector space might admit multiple such norms.)
- A topological space is called a T1 space if, for every two distinct points , there is an open neighbourhood of that does not contain . In a topological vector space, this is equivalent to requiring that, for every , there is an open neighbourhood of the origin not containing . Note that being T1 is weaker than being a Hausdorff space, in which every two distinct points admit open neighbourhoods of and of with ; since normed and normable spaces are always Hausdorff, it is a “surprise” that the theorem only requires T1.
- A subset of a vector space is a convex set if, for any two points , the line segment joining them lies wholly within , i.e., for all , .
- A subset of a topological vector space is a bounded set if, for every open neighbourhood of the origin, there exists a scalar so that . (One can think of as being “small” and as being “big enough” to inflate to cover .)
Expressed in these terms, Kolmogorov's normability criterion is as follows:
Theorem. A topological vector space is normable if and only if it is a T1 space and admits a bounded convex neighbourhood of the origin.