Buchholz's ordinal is a large countable ordinal used in the area of proof theory and mathematical logic. It is named after Wilhelm Buchholz, who introduced it as part of his work on subsystems of second order arithmetic and their provable ordinals. Buchholz's ordinal is often denoted as \( \epsilon_0^{\#} \) and is significant in the study of proof-theoretic strength of various formal systems.
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