Counting quantification: Difference between revisions

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Counting quantifiers can be defined [[recursive definition|recursively]] in terms of ordinary quantifiers.

Let <math>\exists^exists_{= k}</math> denote "there exist exactly <math>k</math>". Then

:<math>\begin{align}

\exists^exists_{= 0} x P x &\leftrightarrow \neg \exists x P x \\

\exists^exists_{= k+1} x P x &\leftrightarrow \exists x (P x \land \exists^exists_{= k} y (P y \neqland xy \landneq P yx))

\end{align}</math>

Let <math>\exists^exists_{\geq k}</math> denote "there exist at least <math>k</math>". Then

:<math>\begin{align}

\exists^exists_{\geq 0} x P x &\leftrightarrow \top \\

\exists^exists_{\geq k+1} x P x &\leftrightarrow \exists x (P x \land \exists^exists_{\geq k} y (P y \neqland xy \landneq P yx))

\end{align}</math>

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*[[Uniqueness quantification]]

*[[Lindström quantifier]]

*[[Spectrum of a sentence]]

== References ==