Band (order theory)

{{Multiple issues|{{one source|date=June 2020}}{{inline|date=September 2020}}{{technical|date=June 2020}}}}

{{Distinguish|Band (algebra)}}

In mathematics, specifically in order theory and functional analysis, a band in a vector lattice X is a subspace M of X that is solid and such that for all S \subseteq M such that x = \sup S exists in X, we have x \in M.{{sfn|Narici|Beckenstein|2011|pp=204–214}}

The smallest band containing a subset S of X is called the band generated by S in X.{{sfn|Narici|Beckenstein|2011|pp=204–214}}

A band generated by a singleton set is called a principal band.

Examples

For any subset S of a vector lattice X, the set S^{\perp} of all elements of X disjoint from S is a band in X.{{sfn|Narici|Beckenstein|2011|pp=204–214}}

If \mathcal{L}^p(\mu) (1 \leq p \leq \infty) is the usual space of real valued functions used to define Lp spaces L^p, then \mathcal{L}^p(\mu) is countably order complete (that is, each subset that is bounded above has a supremum) but in general is not order complete.

If N is the vector subspace of all \mu-null functions then N is a solid subset of \mathcal{L}^p(\mu) that is {{em|not}} a band.{{sfn|Narici|Beckenstein|2011|pp=204–214}}

Properties

The intersection of an arbitrary family of bands in a vector lattice X is a band in X.{{sfn | Schaefer |Wolff| 1999 | pp=204–214}}

See also

  • {{annotated link|Solid set}}
  • {{annotated link|Locally convex vector lattice}}
  • {{annotated link|Vector lattice}}

References

{{reflist|group=note}}

{{reflist}}

  • {{Narici Beckenstein Topological Vector Spaces|edition=2}}
  • {{Schaefer Wolff Topological Vector Spaces|edition=2}}

{{Ordered topological vector spaces}}

{{Functional analysis}}

Category:Functional analysis