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  "body": "\n## Summary\n\nIn 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 $\u003c M$). Rathjen uses this to diagonalise over the weakly inaccessible hierarchy.\n",
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