Reflections on a Proof of Elementarity

Robert Goldblatt

School of Mathematical and Computing Sciences
Victoria University
P.O. Box 600
Wellington
New Zealand
Rob.Goldblatt@vuw.ac.nz

Abstract

This is an exposition and analysis of van Benthem's original proof, hitherto unpublished, that if the class of structures (frames) validating a modal formula is closed under elementary equivalence, then it is the class of all models of a single first-order sentence.

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Photo taken by Rob Goldblatt in a Tubingen restaurant in December 1977.
From left to right are Jens Erik Fenstad, Johan van Benthem, Dagfinn Follesdal, Ede Zimmermann, Arnim von Stechow, Uwe Monnich, ??