A New Modal Lindstrom Theorem
Johan van Benthem

Abstract:
We prove new Lindstrom theorems for the basic modal propositional
language, and for the guarded fragment of first-order logic. We find
difficulties with such results for modal languages without a
finite-depth property, high- lighting the difference between abstract
model theory for fragments vs. that for extensions of first-order
logic.

Keywords: modal logic;  bisimulation;  ;  lindström theorem;  abstract;
  model theory;  finite depth property;  ;  guarded fragment