 Rathjen's psi function

Rathjen's psi function is a mathematical function related to proof theory and the foundations of mathematics, particularly in the context of ordinal analysis and proof-theoretic strength. It is primarily associated with the work of the mathematician and logician Michael Rathjen. The psi function is often used in the analysis of certain subsystems of arithmetic and serves as a tool in the study of the relationships between different proof-theoretic systems, including their consistency and completeness properties.