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<p>problems.</p>

<p><big>Defining complexity classes</big></p>
<p>The definitions of the complexity classes <b>NP</b>, <b>PSPACE</b>, and <b>EXPTIME</b> do not involve reductions: reductions come into their study only in the definition of complete languages for these classes. However, in some cases a complexity class may be defined by reductions. If C is any <a href="page.php?w=decision_problem">decision problem</a>, then one can define a complexity class <b>C</b> consisting of the languages A for which . In this case, C will automatically be complete for</p><p>
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