# Luzin hypothesis

*in set theory*

The cardinality of the continuum is the cardinality of the set of all subsets of the countable ordinals, that is, $2^{\aleph_0}=2^{\aleph_1}$. Luzin's hypothesis is compatible with the Zermelo–Fraenkel system of axioms of set theory and the axiom of choice. N.N. Luzin [1] considered this hypothesis as an alternative to the continuum hypothesis, that is, $2^{\aleph_0}=\aleph_1<2^{\aleph_1}$. Martin's axiom (cf. Suslin hypothesis) and the negation of the continuum hypothesis together imply the Luzin hypothesis. The negation of the Luzin hypothesis, $2^{\aleph_0}<2^{\aleph_1}$, is also sometimes called the Luzin hypothesis. The Luzin hypothesis, denoted by (HL), or its negation, which is denoted by (LH), are used in the proof of a number of theorems in general topology. For example, (LH) is equivalent to one of the following assertions: any compact space of cardinality not exceeding the cardinality of the continuum has an everywhere-dense subspace that satisfies the first axiom of countability; any dyadic compact Hausdorff space of cardinality not exceeding the cardinality of the continuum is metrizable. The following propositions follow from (LH): any normal space that satisfies the first axiom of countability and the Suslin condition is collection-wise normal; any separable normal Moore space is metrizable.

#### References

[1] | N.N. [N.N. Luzin] Lusin, "Sur les ensembles analytiques nuls" Fund. Math. , 25 (1935) pp. 109–131 |

[2] | A. Mostowski, "Constructible sets ands applications" , North-Holland (1969) |

#### Comments

For the consistency of Luzin's hypothesis see also [a1].

#### References

[a1] | K. Kunen, "Set theory" , North-Holland (1980) |

**How to Cite This Entry:**

Luzin hypothesis.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Luzin_hypothesis&oldid=32793