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Abstract$ε$-terms, introduced by David Hilbert [2], have the form $ε x.φ$, where x is a variable and $φ$ is a formula. Their syntactical structure is thus similar to that of a quantified formulae, but they are terms, denoting `an element for which $φ$ holds, if there is any'. The topic of this paper is an investigation into the possibilities and limits of using $ε$-terms for automated theorem proving. We discuss the relationship between $ε$-terms and Skolem terms (which both can be used alternatively for the purpose of $∃$-quantifier elimination), in particular with respect to efficiency and intuition. We also discuss the consequences of allowing $ε$-terms in theorems (and cuts). This leads to a distinction between (essentially two) semantics and corresponding calculi, one enabling efficient automated proof search, and the other one requiring human guidance but enabling a very intuitive (i.e. semantic) treatment of $ε$-terms. We give a theoretical foundation of the usage of both variants in a single framework. Finally, we argue that these two approaches to $ε$ are just the extremes of a range of $ε$-treatments, corresponding to a range of different possible Skolemization variants.
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