Admissibility and Constraint-Induced Rigidity of Physical Quotient Structure (Structural Unification)

Admissible Equivalence Rigidity (AER): Parameter-Free Closure and Structural Determinacy Abstract / Description: This paper develops and formalizes the Admissible Equivalence Rigidity (AER) framework: a structural rigidity theorem for equivalence relations defined under parameter-free admissibility constraints. The central result shows that when admissibility is enforced as a closure-preserving condition—excluding external parameters, hidden selectors, or post-hoc transport—admissible equivalence classes exhibit rigidity: their structure is fixed by internal consistency and cannot be continuously deformed without violating admissibility. The manuscript provides: A precise formal definition of admissible equivalence under closure constraints. A rigidity theorem demonstrating that admissible equivalence classes are structurally determined (no free moduli under admissible redescription). A strengthened proof eliminating selector ambiguity and deferred admissibility vulnerabilities. Explicit examples on smooth manifolds and constraint-defined strata illustrating the rigidity mechanism. Discussion of limitations (e.g., singular strata, non-smooth quotients) and the scope of the parameter-free regime. Conceptually, the work isolates a purely structural phenomenon: once external parameterization is prohibited, equivalence collapses to a uniquely determined interior form. This places admissible rigidity in conversation with geometric rigidity theory, constraint closure in mathematical physics, and foundational studies of parameter elimination. The result is not dynamical and does not rely on perturbative or variational methods. It is structural and closure-based: admissibility functions as a non-negotiable boundary condition, and rigidity emerges as a necessary consequence. This paper is part of a broader arc examining structural determinacy under closure constraints.