Rathjen's psi function

Summary

In mathematics, Rathjen's $\psi$ psi function is an ordinal collapsing function developed by Michael Rathjen. It collapses weakly Mahlo cardinals $M$ to generate large countable ordinals. A weakly Mahlo cardinal is a cardinal such that the set of regular cardinals below $M$ is closed under $M$ (i.e. all normal functions closed in $M$ are closed under some regular ordinal $< M$). Rathjen uses this to diagonalise over the weakly inaccessible hierarchy.