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A formal logic *(term). See _("the Stanford Encyclopedia of Philosophy's entry" |http://plato.stanford.edu/entries/epsilon-calculus/).

In classical logic, an &epsilon;-term <code>&epsilon;(x) A(x)</code> denotes some value <code>x<sub>0</sub></code> that satisfies <code>A(x<sub>0</sub>) &harr; &exist;(x) A(x)</code>.

Note that this is different from &exist; in that it answers a satisfactory object (a "witness") instead of just the boolean answer to the query.

In some constructive logic, the &epsilon;-term will converge to a witness if and only if a witness exists, and otherwise will diverge (fail to yield a value).

In a deep sense, &epsilon; is to &exist; what &lambda; is to &forall;.


<table><th>Legenda</th>
<tr><td>&epsilon;</td><td>epsilon</td></tr>
<tr><td>&lambda;</td><td>lambda</td></tr>
<tr><td>&harr;</td><td>if and only if</td></tr>
<tr><td>&forall;</td><td>for all</td></tr>
<tr><td>&exist;</td><td>there exists</td></tr>
</table>

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