Totally bounded space
In
The term precompact (or pre-compact) is sometimes used with the same meaning, but precompact is also used to mean
In metric spaces
A metric space is totally bounded if and only if for every real number , there exists a finite collection of
Each totally bounded space is
Uniform (topological) spaces
A metric appears in the definition of total boundedness only to ensure that each element of the finite cover is of comparable size, and can be weakened to that of a
The definition can be extended still further, to any category of spaces with a notion of
Examples and elementary properties
- Every compact setis totally bounded, whenever the concept is defined.
- Every totally bounded set is bounded.
- A subset of the real line, or more generally of finite-dimensional Euclidean space, is totally bounded if and only if it is bounded.[5][3]
- The dimension.
- Equicontinuous bounded functions on a compact set are precompact in the uniform topology; this is the Arzelà–Ascoli theorem.
- A metric space is separable if and only if it is homeomorphic to a totally bounded metric space.[3]
- The closure of a totally bounded subset is again totally bounded.[6]
Comparison with compact sets
In metric spaces, a set is compact if and only if it is complete and totally bounded;[5] without the axiom of choice only the forward direction holds. Precompact sets share a number of properties with compact sets.
- Like compact sets, a finite union of totally bounded sets is totally bounded.
- Unlike compact sets, every subset of a totally bounded set is again totally bounded.
- The continuous image of a compact set is compact. The uniformly continuous image of a precompact set is precompact.
In topological groups
Although the notion of total boundedness is closely tied to metric spaces, the greater algebraic structure of topological groups allows one to trade away some separation properties. For example, in metric spaces, a set is compact if and only if complete and totally bounded. Under the definition below, the same holds for any topological vector space (not necessarily Hausdorff nor complete).[6][7][8]
The general
We adopt the convention that, for any neighborhood of the identity, a subset is called (left) -small if and only if [6] A subset of a topological group is (left) totally bounded if it satisfies any of the following equivalent conditions:
- Definition: For any neighborhood of the identity there exist finitely many such that
- For any neighborhood of there exists a finite subset such that (where the right hand side is the Minkowski sum).
- For any neighborhood of there exist finitely many subsets of such that and each is -small.[6]
- For any given filter subbase of the identity element's neighborhood filter(which consists of all neighborhoods of in ) and for every there exists a cover of by finitely many -small subsets of [6]
- is Cauchy bounded: for every neighborhood of the identity and every countably infinite subset of there exist distinct such that [6] (If is finite then this condition is satisfied vacuously).
- Any of the following three sets satisfies (any of the above definitions of) being (left) totally bounded:
- The closure of in [6]
- This set being in the list means that the following characterization holds: is (left) totally bounded if and only if is (left) totally bounded (according to any of the defining conditions mentioned above). The same characterization holds for the other sets listed below.
- The image of under the canonical quotientwhich is defined by (where is the identity element).
- The sum [9]
- The closure of in [6]
The term pre-compact usually appears in the context of Hausdorff topological vector spaces.[10][11] In that case, the following conditions are also all equivalent to being (left) totally bounded:
- In the completion of the closure of is compact.[10][12]
- Every ultrafilter on is a Cauchy filter.
The definition of right totally bounded is analogous: simply swap the order of the products.
Condition 4 implies any subset of is totally bounded (in fact, compact; see § Comparison with compact sets above). If is not Hausdorff then, for example, is a compact complete set that is not closed.[6]
Topological vector spaces
Any topological vector space is an abelian topological group under addition, so the above conditions apply. Historically, statement 6(a) was the first reformulation of total boundedness for topological vector spaces; it dates to a 1935 paper of John von Neumann.[13]
This definition has the appealing property that, in a
For separable Banach spaces, there is a nice characterization of the precompact sets (in the norm topology) in terms of weakly convergent sequences of functionals: if is a separable Banach space, then is precompact if and only if every weakly convergent sequence of functionals converges uniformly on [14]
Interaction with convexity
- The balanced hull of a totally bounded subset of a topological vector space is again totally bounded.[6][15]
- The Minkowski sumof two compact (totally bounded) sets is compact (resp. totally bounded).
- In a locally convex (Hausdorff) space, the disked hullof a totally bounded set is totally bounded if and only if is complete.[16]
See also
- Compact space
- Locally compact space
- Measure of non-compactness
- Orthocompact space
- Paracompact space
- Relatively compact subspace
References
- ^ Sutherland 1975, p. 139.
- ^ "Cauchy sequences, completeness, and a third formulation of compactness" (PDF). Harvard Mathematics Department.
- ^ a b c Willard 2004, p. 182.
- ^ Willard, Stephen (1970). Loomis, Lynn H. (ed.). General topology. Reading, Mass.: Addison-Wesley. p. 262. C.f. definition 39.7 and lemma 39.8.
- ^ a b Kolmogorov, A. N.; Fomin, S. V. (1957) [1954]. Elements of the theory of functions and functional analysis,. Vol. 1. Translated by Boron, Leo F. Rochester, N.Y.: Graylock Press. pp. 51–3.
- ^ a b c d e f g h i Narici & Beckenstein 2011, pp. 47–66.
- ^ Narici & Beckenstein 2011, pp. 55–56.
- ^ Narici & Beckenstein 2011, pp. 55–66.
- ^ Schaefer & Wolff 1999, pp. 12–35.
- ^ a b Schaefer & Wolff 1999, p. 25.
- ^ Trèves 2006, p. 53.
- ^ Jarchow 1981, pp. 56–73.
- ISSN 0002-9947.
- ^ Phillips, R. S. (1940). "On Linear Transformations". Annals of Mathematics: 525.
- ^ Narici & Beckenstein 2011, pp. 156–175.
- ^ Narici & Beckenstein 2011, pp. 67–113.
Bibliography
- Jarchow, Hans (1981). Locally convex spaces. Stuttgart: B.G. Teubner. OCLC 8210342.
- Narici, Lawrence; Beckenstein, Edward (2011). Topological Vector Spaces. Pure and applied mathematics (Second ed.). Boca Raton, FL: CRC Press. OCLC 144216834.
- OCLC 840278135.
- Zbl 0304.54002.
- OCLC 853623322.
- Willard, Stephen (2004). General Topology. Dover Publications. ISBN 0-486-43479-6.