Interpolation for extended modal languages

Several extensions of the basic modal language are characterized in terms of interpolation. Our main results are of the following form: Language L' is the least expressive extension of L with interpolation. For instance, let M (D) be the extension of the basic modal language with a difference operator [7]. First-order logic is the least expressive extension of M (D) with interpolation. These characterizations are subsequently used to derive new results about hybrid logic. relation algebra and the guarded fragment.