Operations and equations for coalgebras

  • Authors:
  • Alexander Kurz;Jiří Rosický

  • Affiliations:
  • Department of Computer Science, University of Leicester, UK;Masaryk University, Janáčkovo nám. 2a, 662 95 Brno, Czech Republic

  • Venue:
  • Mathematical Structures in Computer Science
  • Year:
  • 2005

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Abstract

We show how coalgebras can be presented by operations and equations. This is a special case of Linton's approach to algebras over a general base category ${\cal X}$, namely where ${\cal X}$ is taken as the dual of sets. Since the resulting equations generalise coalgebraic coequations to situations without cofree coalgebras, we call them coequations. We prove a general co-Birkhoff theorem describing covarieties of coalgebras by means of coequations. We argue that the resulting coequational logic generalises modal logic. This relies on the fact that coalgebraic operations respect an appropriate notion of bisimulation and can be considered as modal operators.