Aronszajn line

In mathematical set theory, an Aronszajn line (named after Nachman Aronszajn) is a linear ordering of cardinality [math]\displaystyle{ \aleph_1 }[/math] which contains no subset order-isomorphic to

• [math]\displaystyle{ \omega_1 }[/math] with the usual ordering
• the reverse of [math]\displaystyle{ \omega_1 }[/math]
• an uncountable subset of the Real numbers with the usual ordering.

Unlike Suslin lines, the existence of Aronszajn lines is provable using the standard axioms of set theory. A linear ordering is an Aronszajn line if and only if it is the lexicographical ordering of some Aronszajn tree.