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A Proof Pearl with the Fan Theorem and Bar Induction: Walking through infinite trees with mixed induction and coinduction

Resumo: We study temporal properties over infinite binary red-blue trees in the setting of constructive type theory. We consider several familiar path-based properties, typical to linear-time and branching-time temporal logics like LTL and CTL*, and the corresponding tree-based properties, in the spirit of the modal mu-calculus. We conduct a systematic study of the relationships of the path-based and tree-based versions of "eventually redness", "eventually always blueness" and mixed inductive-coinductive "almost always blueness". We relate these properties to each other in terms of implications that hold either unconditionally or under specific assumptions (Weak Continuity for Numbers, the Fan Theorem, Lesser Principle of Omniscience, Bar Induction). We have fully formalized our development with the Coq proof assistant. (Joint work with Marc Bezem and Tarmo Uustalu)
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