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Department of Mathematics
Stanford University
Stanford
CA 94305-2125 USA
sf@csli.stanford.edu
http://math.Stanford.EDU/~feferman/
A theorem obtained by van Benthem for preservation of formulas under Chu transforms between Chu spaces is strengthened and derived from a general many-sorted interpolation theorem. The latter has been established both by proof-theoretic and model-theoretic methods; there is some discussion as to how these methods compare and what languages they apply to. In the conclusion, several further questions are raised.