Jech–Kunen tree
# Jech–Kunen tree

A Jech–Kunen tree is a set-theoretic tree with properties that are incompatible with the generalized continuum hypothesis. It is named after Thomas Jech and Kenneth Kunen, both of whom studied the possibility and consequences of its existence.

## Definition

A $ω_1-tree$ is a tree with cardinality $\aleph _{1}$ and height $ω_1,$ where $ω_1$ is the first uncountable ordinal and $\aleph _{1}$ is the associated cardinal number. A Jech–Kunen tree is a $ω_1-tree$ in which the number of branches is greater than $\aleph _{1}$ and less than $2^{\aleph _{1}}$.

## Existence

Thomas Jech (1971) found the first model in which this tree exists, and Kenneth Kunen (1975) showed that, assuming the continuum hypothesis and $2^{\aleph _{1}}>\aleph _{2}$ , the existence of a Jech–Kunen tree is equivalent to the existence of a compact Hausdorff space with weight $\aleph _{1}$ and cardinality strictly between $\aleph _{1}$ and $2^{\aleph _{1}}$.

