A monohedron is a convex polyhedron with all faces mutually congruent but with no other symmetry necessarily needed. So obviously, this is a wide class of polyhedrons that includes the Platonic solids and isohedra (https://mathworld.wolfram.com/Isohedron.html). An earlier post is https://math.stackexchange.com/questions/3888486/what-are-the-known-convex-polyhedra-with-congruent-faces

Questions: Are there monohedrons with odd number of faces (it is known that isohedrons necessarily have even number of faces - as stated in https://mathworld.wolfram.com/Isohedron.html)? What are the values for the number of edges on a face for which monohedrons are possible? Will relaxing convexity (of the body, not of the faces) have an impact on the answers to these questions?

  • $\begingroup$ This violates a few of your conditions, but you may find the 7-faced Szilassi polyhedron interesting: en.wikipedia.org/wiki/Szilassi_polyhedron $\endgroup$ Oct 13 '21 at 12:52
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    $\begingroup$ Considering the dual of the edge-graph, using the hand-shaking-lemma and that there must be a vertex of degree at most 5, we see that the only option for an odd number of faces is if the monohedron has quadrangle faces. The faces can also not be centrally symmetric as otherwise the whole polyhedron would be a centrally symmetric zonohedron, having even many faces. $\endgroup$
    – M. Winter
    Oct 13 '21 at 13:04
  • $\begingroup$ Thanks Bill Bradley for the intro to Szilassi polyhedron. I hadn't thought about polyhedra that are not simply connected. But this is a very interesting object - although the faces are not congruent! And thanks M Winter for your analysis; however, I am unable to visualize or draw a polyhedron with odd number of faces and all faces being mutually congruent and non-centrally symmetric quadrangles. $\endgroup$ Oct 14 '21 at 8:27
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    $\begingroup$ To add to M. Winter's comment: the four edges of the face cannot include an edge whose length is unique, because then pairs of faces would have to be matched across these edges and the total number of faces would have to be even. Together with the impossibility of centrally symmetric faces that means that the only possible face shapes are kites. $\endgroup$ Oct 17 '21 at 8:01
  • $\begingroup$ Thanks for pointing this out, Prof Eppstein. $\endgroup$ Oct 17 '21 at 16:37

This is too long for a comment.

The figure shows how to construct an $11$-face convex polyhedron whose faces all quadrilaterals, but not congruent. I think it's only interesting because it shows that there aren't plain combinatorial or topological obstructions to a positive answer to the question. On the other hand I doubt it's possible to deform this specific construction to produce a monohedron (I don't see how to make all the faces have the same number and order of obtuse and acute angles, never mind corresponding edges of equal length).

Start with a hexagonal prism. Cut the top hexagon in $2$ and the bottom one in $3$, to created $5$ degenerate (coplanar) quadrangular faces in place of the two hexagons. Slide G, L and M out a bit to make the faces not coplanar, but since this may destroy the flatness, slide H, I, J, K, C, E, B in a bit, each as necessary, to make all the faces flat again. enter image description here

  • $\begingroup$ Nice! Guess this is at least a very strong indication that monohedra with odd number of faces exist - maybe the number of faces may be considerably more than 11. And whether simply connectedness and convexity (lack thereof) have anything to do with the question would also hopefully have interesting answers. $\endgroup$ Oct 17 '21 at 5:35
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    $\begingroup$ See also en.wikipedia.org/wiki/Herschel_graph for a simpler 9-face example of a polyhedron with an odd number of (non-congruent) quadrilateral faces. $\endgroup$ Oct 17 '21 at 7:47
  • $\begingroup$ @DavidEppstein Following up on your remark that all faces need to be kites: this means that lengths come in 2 types (S, "short", and L, "long") and opposite edges must have different types. Both Herschel's graph and the 11-face example here have the property that the all edges can be colored S or L in such a way; the coloring is unique (up to permutations) for Herschel's graph, and comes in 2 flavors for the 11-face. $\endgroup$ Nov 17 '21 at 11:56
  • $\begingroup$ However in both the Herschel and and the two colorings of the 11-face graph, there are vertices containing all four angles of the kite, which adds up to $2\pi$ and is therefore not possible in a convex polyhedron. So an example, if it exists, will have to come from more complex figures. $\endgroup$ Nov 17 '21 at 13:22

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