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<p>the problem of finding proofs (automated theorem proving) is usually <a href="page.php?w=Computationally_intractable">computationally intractable</a> and/or only <a href="page.php?w=semi-decidable">semi-decidable</a>, depending upon the formal system in use.</p>

<p><big> Background </big></p>
<p><big> Formal language </big></p>
<p>A formal language is a <a href="page.php?w=Set_%28mathematics%29">set</a> of finite <a href="page.php?w=sequence_%28mathematics%29">sequences</a> of <a href="page.php?w=symbol">symbol</a>s. Such a language can be defined without <a href="page.php?w=reference">reference</a></p><p>
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