Menger space explained

In mathematics, a Menger space is a topological space that satisfies a certain basic selection principle that generalizes σ-compactness. A Menger space is a space in which for every sequence of open covers

l{U}1,l{U}2,\ldots

of the space there are finite sets

l{F}1\subsetl{U}1,l{F}2\subsetl{U}2,\ldots

such that the family

l{F}1\cupl{F}2\cup

covers the space.

History

In 1924, Karl Menger [1] introduced the following basis property for metric spaces: Every basis of the topology contains a countable family of sets with vanishing diameters that covers the space. Soon thereafter, Witold Hurewicz [2] observed that Menger's basis property can be reformulated to the above form using sequences of open covers.

Menger's conjecture

Menger conjectured that in ZFC every Menger metric space is σ-compact. A. W. Miller and D. H. Fremlin[3] proved that Menger's conjecture is false, by showing that there is,in ZFC, a set of real numbers that is Menger but not σ-compact. The Fremlin-Miller proof was dichotomic, and the set witnessing the failureof the conjecture heavily depends on whether a certain (undecidable) axiomholds or not.

Bartoszyński and Tsaban[4] gave a uniform ZFC example of a Menger subset of the real line that is not σ-compact.

Combinatorial characterization

NN

.For functions

f,g\inNN

, write

f\leq*g

if

f(n)\leqg(n)

for all but finitely many natural numbers

n

. A subset

A

of

NN

is dominating if for each function

f\inNN

there is a function

g\inA

such that

f\leq*g

. Hurewicz proved that a subset of the real line is Menger iff every continuous image of that space into the Baire space is not dominating. In particular, every subset of the real line of cardinality less than the dominating number

ak{d}

is Menger.

The cardinality of Bartoszyński and Tsaban's counter-example to Menger's conjecture is

ak{d}

.

Properties

F\sigma

subsets

Notes and References

  1. Book: Menger. Karl. Einige Überdeckungssätze der Punktmengenlehre . Selecta Mathematica . Sitzungsberichte der Wiener Akademie. 1924. 133. 421–444. 10.1007/978-3-7091-6110-4_14. 978-3-7091-7282-7.
  2. Hurewicz. Witold. Über eine verallgemeinerung des Borelschen Theorems. Mathematische Zeitschrift. 1926. 24. 1 . 401–421. 10.1007/bf01216792. 119867793 .
  3. Fremlin. David. Miller. Arnold. On some properties of Hurewicz, Menger and Rothberger. Fundamenta Mathematicae. 1988. 129. 17–33. 10.4064/fm-129-1-17-33 .
  4. Bartoszyński. Tomek. Tsaban. Boaz. Hereditary topological diagonalizations and the Menger–Hurewicz Conjectures. Proceedings of the American Mathematical Society. 2006. 134. 2. 605–615. 10.1090/s0002-9939-05-07997-9. math/0208224. 9931601 .
  5. Chodounský. David. Repovš. Dušan. Zdomskyy. Lyubomyr. Mathias Forcing and Combinatorial Covering Properties of Filters . 2015-12-01. The Journal of Symbolic Logic. 80. 4. 1398–1410. 10.1017/jsl.2014.73. 0022-4812. 1401.2283. 15867466 .