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Essential spectrum
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<p>In <a href="/facts/Mathematics/pxTouaz4">mathematics</a>, the essential spectrum of a <a href="/facts/Bounded_operator/LYmDTIqR">bounded operator</a> (or, more generally, of a <a href="/facts/Densely_defined/8uGARqJR">densely defined</a> <a href="/facts/Unbounded_operator/5u2njqHt">closed linear operator</a>) is a certain subset of its <a href="/facts/Spectrum_(functional_analysis)/cA1evTo6">spectrum</a>, defined by a condition of the type that says, roughly speaking, "fails badly to be invertible". </p>