In mathematics, specifically in the field of topology, a monotonically normal space is a particular kind of normal space, defined in terms of a monotone normality operator. It satisfies some interesting properties; for example metric spaces and linearly ordered spaces are monotonically normal, and every monotonically normal space is hereditarily normal.

Definition

A topological space is called monotonically normal if it satisfies any of the following equivalent definitions:[1][2][3][4]

Definition 1

The space is T1 and there is a function that assigns to each ordered pair of disjoint closed sets in an open set such that:

(i) ;
(ii) whenever and .

Condition (i) says is a normal space, as witnessed by the function . Condition (ii) says that varies in a monotone fashion, hence the terminology monotonically normal. The operator is called a monotone normality operator.

One can always choose to satisfy the property

,

by replacing each by .

Definition 2

The space is T1 and there is a function that assigns to each ordered pair of separated sets in (that is, such that ) an open set satisfying the same conditions (i) and (ii) of Definition 1.

Definition 3

The space is T1 and there is a function that assigns to each pair with open in and an open set such that:

(i) ;
(ii) if , then or .

Such a function automatically satisfies

.

(Reason: Suppose . Since is T1, there is an open neighborhood of such that . By condition (ii), , that is, is a neighborhood of disjoint from . So .)[5]

Definition 4

Let be a base for the topology of . The space is T1 and there is a function that assigns to each pair with and an open set satisfying the same conditions (i) and (ii) of Definition 3.

Definition 5

The space is T1 and there is a function that assigns to each pair with open in and an open set such that:

(i) ;
(ii) if and are open and , then ;
(iii) if and are distinct points, then .

Such a function automatically satisfies all conditions of Definition 3.

Examples

  • Every metrizable space is monotonically normal.[4]
  • Every linearly ordered topological space (LOTS) is monotonically normal.[6][4] This is assuming the Axiom of Choice, as without it there are examples of LOTS that are not even normal.[7]
  • The Sorgenfrey line is monotonically normal.[4] This follows from Definition 4 by taking as a base for the topology all intervals of the form and for by letting . Alternatively, the Sorgenfrey line is monotonically normal because it can be embedded as a subspace of a LOTS, namely the double arrow space.
  • Any generalised metric is monotonically normal.

Properties

  • Monotone normality is a hereditary property: Every subspace of a monotonically normal space is monotonically normal.
  • Every monotonically normal space is completely normal Hausdorff (or T5).
  • Every monotonically normal space is hereditarily collectionwise normal.[8]
  • The image of a monotonically normal space under a continuous closed map is monotonically normal.[9]
  • A compact Hausdorff space is the continuous image of a compact linearly ordered space if and only if is monotonically normal.[10][3]

References

  1. Heath, R. W.; Lutzer, D. J.; Zenor, P. L. (April 1973). "Monotonically Normal Spaces" (PDF). Transactions of the American Mathematical Society. 178: 481–493. doi:10.2307/1996713. JSTOR 1996713.
  2. Borges, Carlos R. (March 1973). "A Study of Monotonically Normal Spaces" (PDF). Proceedings of the American Mathematical Society. 38 (1): 211–214. doi:10.2307/2038799. JSTOR 2038799.
  3. 1 2 Bennett, Harold; Lutzer, David (2015). "Mary Ellen Rudin and monotone normality" (PDF). Topology and Its Applications. 195: 50–62. doi:10.1016/j.topol.2015.09.021.
  4. 1 2 3 4 Brandsma, Henno. "monotone normality, linear orders and the Sorgenfrey line". Ask a Topologist.
  5. Zhang, Hang; Shi, Wei-Xue (2012). "Monotone normality and neighborhood assignments" (PDF). Topology and Its Applications. 159 (3): 603–607. doi:10.1016/j.topol.2011.10.007.
  6. Heath, Lutzer, Zenor, Theorem 5.3
  7. van Douwen, Eric K. (September 1985). "Horrors of Topology Without AC: A Nonnormal Orderable Space" (PDF). Proceedings of the American Mathematical Society. 95 (1): 101–105. doi:10.2307/2045582. JSTOR 2045582.
  8. Heath, Lutzer, Zenor, Theorem 3.1
  9. Heath, Lutzer, Zenor, Theorem 2.6
  10. Rudin, Mary Ellen (2001). "Nikiel's conjecture" (PDF). Topology and Its Applications. 116 (3): 305–331. doi:10.1016/S0166-8641(01)00218-8.
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