Abstract :
Inexpressibility results in Finite Model Theory are often proved by showing that Duplicator, one of the two players of an Ehrenfeucht game, has a winning strategy on certain structures.
In this article a new method is introduced that allows, under certain conditions, the extension of a winning strategy of Duplicator on some small parts of two finite structures to a global winning strategy.
As applications of this technique it is shown that
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— Graph Connectivity is not expressible in existential monadic second-order logic (MonNP), even in the presence of a built-in linear order,
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— Graph Connectivity is not expressible in MonNP even in the presence of arbitrary built-in relations of degree n0(1), and
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— the presence of a built-in linear order gives MonNP more expressive power than the presence of a built-in successor relation.
