Spahn algebraic- and coalgebraic semantics of modal logic (Rev #4)

This entry is about

• Yde Venema, Algebras and Coalgebras, §6 (p.332-426)in Blackburn, van Benthem, Wolter, Handbook of modal logic, Elsevier, 2007.

The bibliography lists many articles of Robert Goldblatt and Bart Jacobs on modal logic.

1 Introduction

(p.332)

(…)

An important difference of the coalgebraic approach compared to the algebraic one is that coalgebras generalize rather than dualize the model theory of modal logic. One may generalize the concept of modal logic from Kripke frames to arbitrary coalgebras. In fact the link between modal logic and coalgebra is so strong that one may claim that modal logic is the natural logic for coalgebras (just like equational logic is that for algebra)

9 Coalgebras: an introduction

(p.389)

The notion of coalgebra was formed in the sixties but the topic attracted a wider attention only when was realized that coalgebras can be seen as a general uniform theory of dynamic systems.

We list some applications (or examples) of the theory of coalgebras.

• The main examples for coalgebras are Kripke frames.

• Aczel in [non-well founded sets] models transition systems and non-well fondet sets as coalgebras.

• Barwise-Moss in [Vicious Circles] discuss notions of circularity and self-reference (and many (other) applications).

• Rutten [Automata and coinduction] on coalgebras and deterministic automata.

(Definition 146, p.392) A polynomial functor is a $Set$-valued functor inductively defined by

where $I$ denotes the identity functor, $C$ denotes the constant functor on an object $C$, $K_0 +K_1$ denotes the coproduct functor of two polynomial functors, and likewise for the product functor, $K^D$ denotes the exponent functor $X\mapsto K(X)^D$.

A Kripke polynomial functor is inductively defined by

where $P K$ denotes the composition of a polynomial functor with the power set functor.