The claim that the accessibility relation in modal logic S5 partitions the domain into equivalence classes is a foundational definition of the semantics for S5 and requires no citation.
The accessibility relation in modal logic S5 forms an equivalence relation that partitions the domain into equivalence classes, without a specific fixed number.
The claim defines standard Kripke semantics for modal logic S5, where an accessibility relation that is an equivalence relation (reflexive, symmetric, and transitive) partitions the set of possible worlds into equivalence classes. This is a matter of mathematical definition and common knowledge in logic, rendering citation of empirical or specialized literature unnecessary.