If the section is perpendicular to a " bisectrix " ( see Crystallography ) a black cross is seen which on rotation opens out to form two hyperbolas, the apices of which are turned towards one another.
2.
:For each pair of vertices ( A, B ), the bisectrix hyperplane divides hyperspace into the points closer to A and closer to B . Therefore there is a bijection between the permutations of the vertices and the chunks into which the hyperplanes partition space.
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