Abstract
In this chapter we make a case for the monadic second-order logic (that is to say, for the extension of first-order logic allowing quantification over monadic predicates) as a good source of theories that are both expressive and manageable. We illustrate two powerful decidability techniques here. One makes use of automata and games. The other is an offshot of a composition theory where one composes models as well as their theories. Monadic second-order logic appears to be the most natural match for the composition theory. Undecidability proofs must be thought out anew in this area; for, whereas true first-order arithmetic is reducible to the monadic theory of the real line R, it is nevertheless not interpretable in the monadic theory of R. A quite unusual undecidability method is another subject of this chapter. In the last section we briefly review the history of the methods thus far developed and mention numerous results obtained by the methods.
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