The truncated tetrahedron covers the tetrahedron

At first, you might think this is obvious – just “clip” off each corner of the tetrahedron \Gamma_1 to create the truncated tetrahedron \Gamma_2 (by essentially creating a triangle from each of these clipped corners – see below for the associated graph). Then just map each such triangle to the corresponding vertex of the tetrahedron. No, it’s not obvious because the map just described is not a covering. This post describes one way to think about how to construct any covering.

First, color the vertices of the tetrahedron in some way.


The coloring below corresponds to a harmonic morphism \phi : \Gamma_2\to \Gamma_1:


All others are obtained from this by permuting the colors. They are all covers of \Gamma_1 = K_4 – with no vertical multiplicities and all horizontal multiplicities equal to 1. These 24 harmonic morphisms of \Gamma_2\to\Gamma_1 are all coverings and there are no other harmonic morphisms.

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