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{\bf Branko Gr\"{u}nbaum and Jonathan Lenchner}
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{\bf Omittable Planes}
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In analogy to omittable lines in the plane, we initiate the study of omittable
planes in $3$-space. Given a collection of $n$ planes in real projective $3$-space, a plane $\Pi$ is said to be omittable if
$\Pi$ is free of ordinary lines of intersection -- in other words, if all the lines of intersection of $\Pi$ with other planes from the collection come at the intersection of three or more planes. We provide two infinite families of planes yielding omittable planes in either a pencil or near-pencil, together with examples having between three and seven omittable planes, examples that we call ``sporadic,'' which do not fit into either of the two infinite families.
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