Logic Seminar

James CummingsCarnegie Mellon University
Squares, scales and lines

Friday, October 17, 2025 - 2:55pm
Malott 205

We use hypotheses from PCF theory to construct a linear ordering which has cardinality the successor of a singular cardinal of countable cofinality, and is "incompact" in the following sense: the ordering is not sigma-scattered, but every smaller subordering is $\sigma$-wellordered. Such orderings were first constructed by Todorcevic using Jensen's $\square$-principle.