nLab topological vector space

Idea

A topological vector space, or TVS for short, is a vector space $X$ over a topological field (usually a local field, more often than not the field of real numbers or the field of complex numbers with the usual topology) $k$ (called the ground field) equipped with a topology for which the addition and scalar multiplication maps

$$+: X \times X \to X, \qquad \cdot: k \times X \to X$$

are continuous. The topological vector spaces over a given field form a category TopVect.

Much as a topological group is a group object in Top, so a TVS is the same as a vector space internal to $Top$ … provided that we use the two-sorted notion of vector space $(k, X)$ so that the first sort is interpreted as the topological ground field. (More generally, there is a notion of topological module which is the internalization in $Top$ of the two-sorted notion of module involving a ring sort and a module over that ring; this notion is purely equational and thus interpretable in terms of finite products. Then a TVS is a topological module whose underlying ring is a field.) N.B.: if we internalize the one-sorted notion vector space, in which the ground field is encoded in the Lawvere theory as the set of unary operations, the continuity of the unary operations gives only a continuous map ${|k|} \times X \to X$ where we use the discrete topology on $k$ – which is not what we want.

Like any topological abelian group, a TVS $X$ carries a uniform space structure generated by a basis of entourages (aka vicinities) that correspond to neighborhoods $U$ of $0$:

$$\{(u, v) \in X \times X: u - v \in U\}$$

Thus many uniform notions (uniform continuity, completeness, etc.) carry over to the TVS context. Also from the uniformity (although it is also easy to prove directly), it follows that a TVS is completely regular, and also Hausdorff if and only if it is $T_0$ (see separation axiom). Most authors insist on the $T_0$ condition to rule out degenerate cases, but that prevents the category of TVSes from being topological over Vect. If the TVS $V$ is not Hausdorff, then the subset $V_0$ defined as the intersection of all neighborhoods of zero is a vector subspace of $V$ and the quotient vector space $V/V_0$ is Hausdorff, hence Tihonov (= completely regular Hausdorff).

The condition that scalar multiplication is continuous puts significant constraints on the topology of $X$. For example, if we assume $k$ has a base of compact absorbing neighborhoods of $0$ and $V$ is Hausdorff, then for any non-zero $v \in X$ the function

$$- \cdot v: k \to V$$

maps $k$ homeomorphically onto its image. It can then be shown that $X$ cannot (for instance) be compact (unless it is the zero space and so has no non-zero $v$); a classical theorem along these lines is that $V$ (over a local field) can be locally compact Hausdorff if and only if $V$ is finite-dimensional. (In the non-Hausdorff case, the theorems are that $X$ is compact if and only if its topology is indiscrete and that $X$ is locally compact if and only if it is a finitary direct sum of indiscrete spaces.) On the other hand, a nice property of even infinite-dimensional TVSes is that they are path-connected, or at least so in the classical cases where the ground field is $\mathbb{R}$ or $\mathbb{C}$.

More classical material should be added, particularly on locally convex spaces.

TVSes from a Hilbert space viewpoint

The theory of TVS can be understood as the quest to find the essence of many fundamental theorems of functional analysis of Hilbert spaces (or Banach spaces), namely to find the minimal set of assumptions that are needed for Hilbert space theorems to remain true. Examples of these are:

• the Hahn–Banach Theorem

• the Open Mapping and closed graph theorems

A central rôle in the whole theory plays duality, that is the study of locally convex spaces and their duals. A prominent example is the definition of certain concepts by duality in the theory of Schwartz distributions.

Important subclasses

Topological vector spaces come in many flavours. The following chart provides a first overview (chart originally created and published by Greg Kuperberg on MathOverflow here, current version generated using Graphviz from lctvs dot source):

category: svg

• locally convex spaces: where the Hahn-Banach theorem works (assuming sufficient axioms)

• bornological topological vector spaces: where bounded means continuous

• barrelled topological vector spaces

• sequence space

• stereotype space

Related concepts

• topology of uniform convergence

• topological vector bundle

References

• Nicolas Bourbaki, Topological Vector Spaces, Chapters 1–5, Masson (1981), Springer (2003) [doi:10.1007/978-3-642-61715-7]

• François Trèves, Topological Vector Spaces, Distributions and Kernels (Academic Press, New York, 1967)

The following review gives lots of important examples

• Paul Garrett, Catalogue of Useful Topological Vectorspaces, 2011 (pdf)

See also

• Wikipedia, Topological vector space

category: analysis