A Fréchet space (or T1 spaces) is a topological space X in wich for any two points a, b, there exist a pair of open sets U and V, such that , and , .
More commonly, the term Fréchet space is applied to an unrelated object in functional analysis. It is a natural generalization of the notion of a Banach space, and the majority of theorems about Banach spaces equally well apply to Fréchet spaces. Whereas a Banach space is a complete topological vector space with the topology induced by some norm , a Fréchet space is a complete topological vector space whose topology is defined by a countably infinite family of seminorms. For example, the space is Fréchet, with topological induced by the Ck norms
Note that these are only seminorms, and not honest norms, since | | f | | k,n may be 0 even if f is not.