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<p>not to statements but to <a href="page.php?w=decision_problem">decision problem</a>s, which are countably infinite sets of questions each requiring a yes or no answer. Such a problem is said to be undecidable if there is no <a href="page.php?w=computable_function">computable function</a> that correctly answers every question in the problem set.  The connection between these two is that if a decision problem is undecidable (in the recursion theoretical sense) then there is no consistent, effective <a href="page.php?w=formal_system">formal system</a></p><p>
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