© 1996 by Oxford University Press
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Original Articles |
On Winning Strategies with Unary Quantifiers
Department of Mathematics P.O. Box 4 (Yliopistonkatu 5) 00014 University of Helsinki Finland E-mail: juha.nurmonen{at}helsinki.fi
A combinatorial argument for two finite structures to agree on all sentences with bounded quantifier rank in first-order logic with any set of unary generalized quantifiers is given. It is known that connectivity of finite structures is neither in monadic
11 nor in
ww (Qu), where Qu is the set of all unary generalized quantifiers. Using this combinatorial argument and a combination of second-order Ehrenfeucht-Fraïssé games developed by Ajtai and Fagin, we prove that connectivity of finite structures is not in monadic
11 with any set of unary quantifiers even if sentences are allowed to contain built-in relations of moderate degree. The combinatorial argument is also used to show that no class (if it is not in some sense trivial) of finite graphs defined by forbidden minors is in
ww(Qu). In particular, the class of planar graphs is not in
ww(Qu)
Keywords: Ehrenfeucht-Fraissé games; finite model theory; forbidden minors; generalized quantifiers; monadic
11