In functional analysis and related areas of mathematics, a Montel space, named after Paul Montel, is any topological vector space in which an analog of Montel's theorem holds. Specifically, a Montel space is a barrelled topological vector space where every closed and bounded set is compact. That is, it satisfies the Heine–Borel property.
Many Montel spaces of contemporary interest arise as spaces of test functions for a space of distributions. The space C∞(Ω) of smooth functions on an open set Ω in Rn is a Montel space equipped with the topology induced by the family of seminorms
for n = 1,2,… and K ranges over compact subsets of Ω, and α is a multi-index. Similarly, the space of compactly supported functions in an open set with the final topology of the family of inclusions as K ranges over all compact subsets of Ω. The Schwartz space is also a Montel space.
No infinite-dimensional Banach space is a Montel space, since these cannot satisfy the Heine–Borel property: the closed unit ball is closed and bounded, but not compact.
They have the following properties:
- The strong dual of a Montel space is Montel.
- Montel spaces are reflexive.
- A nuclear quasi-complete barrelled space is Montel.
- Frechet Montel spaces are separable and have a bornological strong dual.
- Hazewinkel, Michiel, ed. (2001), "Montel space", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- Robertson, A.P.; W.J. Robertson (1964). Topological vector spaces. Cambridge Tracts in Mathematics 53. Cambridge University Press. p. 74.
- Schaefer, Helmuth H. (1971). Topological vector spaces. GTM 3. New York: Springer-Verlag. p. 147. ISBN 0-387-98726-6.
- Treves, François (2006). Topological Vector Spaces, Distributions and Kernels. Dover. ISBN 978-0-486-45352-1..
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