The Range of Modal Logic: an Essay in Memory of George Gargov
Johan van Benthem

Abstract:
THE RANGE OF MODAL LOGIC
an essay in memory of George Gargov
Johan van Benthem

George Gargov was an active pioneer in the 'Sofia School' of modal logicians.
Starting in the 1970s, he and his colleagues expanded the scope of the subject
by introducing new modal expressive power, of various innovative kinds.
The aim of this paper is to show some general patterns behind such extensions,
and review some very general results that we know by now, 20 years later.
We concentrate on simulation invariance, decidability, and correspondence.
What seems clear is that 'modal logic' as a genre of logical systems has a 
much wider scope than originally conceived, and that we have not reached its 
limits yet.