Fricke involution is a concept found in the context of modular forms and algebraic geometry, particularly in relation to the study of modular curves. It is a specific type of involution—meaning it is an operation that can be applied twice to return to the original state—defined on the upper half-plane or on modular forms.
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The Fricke matrix normalizes and under the appropriate determinant-normalized slash action. It exchanges the cusps zero and infinity.