Derrick's theorem
The relative strength of two systems of formal logic can be defined via model theory. Specifically, a logic is said to be as strong as a logic if every elementary class in is an elementary class in .[1]
See also
References
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- ↑ Heinz-Dieter Ebbinghaus Extended logics: the general framework in K. J. Barwise and S. Feferman, editors, Model-theoretic logics, 1985 ISBN 0-387-90936-2 page 43