Tree Rewriting Calculi for Strictly Positive Logics

We study strictly positive logics in the language $\mathscr{L}^+$, which constructs formulas from $\top$, propositional variables, conjunction, and diamond modalities. We begin with the base system $\bf K^+$, the strictly positive fragment of polymodal $\bf K$, and examine its extensions obtained by adding axioms such as monotonicity, transitivity, and the hierarchy-sensitive interaction axiom $(\sf J)$, which governs the interplay between modalities of different strengths. The strongest of these systems is the Reflection Calculus ($\bf RC$), which corresponds to the strictly positive fragment of polymodal $\bf GLP$. Our main contribution is a formulation of these logics as tree rewriting systems, establishing both adequacy and completeness through a correspondence between $\mathscr{L}^+$ formulas and inductively defined modal trees. We also provide a normalization of the rewriting process, which has exponential complexity when axiom $(\sf J)$ is absent; otherwise we provide a double-exponential bound. By introducing tree rewriting calculi as practical provability tools for strictly positive logics, we aim to deepen their proof-theoretic analysis and computational applications.

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