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Δ-sets
A model of <inline-formula content-type="math/mathml"> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="ZFC"> <mml:semantics> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mtext>ZFC</mml:mtext> </mml:mrow> <mml:annotation encoding="application/x-tex">{\text {ZFC}}</mml:annotation> </mml:semantics> </mml:math> </inline-formula> is constructed in which there exists a subset of the Moore plane that is countably paracompact but not normal. The method used in the construction is forcing using uncountable sets of …