Definition
Let be a vector space over either theExamples
Minkowski functionals and seminorms
Seminorms on a vector space are intimately tied, via Minkowski functionals, to subsets of that are convex, balanced, and absorbing. Given such a subset of the Minkowski functional of is a seminorm. Conversely, given a seminorm on the sets and are convex, balanced, and absorbing and furthermore, the Minkowski functional of these two sets (as well as of any set lying "in between them") isAlgebraic properties
Every seminorm is a sublinear function, and thus satisfies all properties of a sublinear function, including: *Relationship to other norm-like concepts
Let be a non-negative function. The following are equivalent:Inequalities involving seminorms
If are seminorms on then:Hahn–Banach theorem for seminorms
Seminorms offer a particularly clean formulation of the Hahn–Banach theorem: :If is a vector subspace of a seminormed space and if is a continuous linear functional on then may be extended to a continuous linear functional on that has the same norm as A similar extension property also holds for seminorms: :Proof: Let be the convex hull of Then is an absorbing disk in and so theTopologies of seminormed spaces
Pseudometrics and the induced topology
A seminorm on induces a topology, called the , via the canonical translation-invariant pseudometric ; This topology is Hausdorff if and only if is a metric, which occurs if and only if is a norm. This topology makes into a locally convex pseudometrizable topological vector space that has a bounded neighborhood of the origin and a neighborhood basis at the origin consisting of the following open balls (or the closed balls) centered at the origin: as ranges over the positive reals. Every seminormed space should be assumed to be endowed with this topology unless indicated otherwise. A topological vector space whose topology is induced by some seminorm is called . Equivalently, every vector space with seminorm induces a vector space quotient where is the subspace of consisting of all vectors with Then carries a norm defined by The resulting topology, pulled back to is precisely the topology induced by Any seminorm-induced topology makes locally convex, as follows. If is a seminorm on and call the set the ; likewise the closed ball of radius is The set of all open (resp. closed) -balls at the origin forms a neighborhood basis of convex balanced sets that are open (resp. closed) in the -topology onStronger, weaker, and equivalent seminorms
The notions of stronger and weaker seminorms are akin to the notions of stronger and weaker norms. If and are seminorms on then we say that is than and that is than if any of the following equivalent conditions holds: # The topology on induced by is finer than the topology induced by # If is a sequence in then in implies in # If is a net in then in implies in # is bounded on # If then for all # There exists a real such that on The seminorms and are called if they are both weaker (or both stronger) than each other. This happens if they satisfy any of the following conditions:Normability and seminormability
A topological vector space (TVS) is said to be a (respectively, a ) if its topology is induced by a single seminorm (resp. a single norm). A TVS is normable if and only if it is seminormable and Hausdorff or equivalently, if and only if it is seminormable and T1 (because a TVS is Hausdorff if and only if it is a T1 space). A is a topological vector space that possesses a bounded neighborhood of the origin. Normability of topological vector spaces is characterized by Kolmogorov's normability criterion. A TVS is seminormable if and only if it has a convex bounded neighborhood of the origin. Thus a locally convex TVS is seminormable if and only if it has a non-empty bounded open set. A TVS is normable if and only if it is a T1 space and admits a bounded convex neighborhood of the origin. If is a Hausdorff locally convex TVS then the following are equivalent:Topological properties
Continuity of seminorms
If is a seminorm on a topological vector space then the following are equivalent:Continuity of linear maps
If is a map between seminormed spaces then let If is a linear map between seminormed spaces then the following are equivalent:Generalizations
The concept of in composition algebras does share the usual properties of a norm. A composition algebra consists of anSee also
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ProofsReferences
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