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# Monodromy Groups and Self-Invariance

Published:2009-12-01
Printed: Dec 2009
• Isabel Hubard
• Alen Orbani\'c
• Asia Ivi\'c Weiss
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## Abstract

For every polytope $\mathcal{P}$ there is the universal regular polytope of the same rank as $\mathcal{P}$ corresponding to the Coxeter group $\mathcal{C} =[\infty, \dots, \infty]$. For a given automorphism $d$ of $\mathcal{C}$, using monodromy groups, we construct a combinatorial structure $\mathcal{P}^d$. When $\mathcal{P}^d$ is a polytope isomorphic to $\mathcal{P}$ we say that $\mathcal{P}$ is self-invariant with respect to $d$, or $d$-invariant. We develop algebraic tools for investigating these operations on polytopes, and in particular give a criterion on the existence of a $d$\nobreakdash-auto\-morphism of a given order. As an application, we analyze properties of self-dual edge-transitive polyhedra and polyhedra with two flag-orbits. We investigate properties of medials of such polyhedra. Furthermore, we give an example of a self-dual equivelar polyhedron which contains no polarity (duality of order 2). We also extend the concept of Petrie dual to higher dimensions, and we show how it can be dealt with using self-invariance.
 Keywords: maps, abstract polytopes, self-duality, monodromy groups, medials of polyhedra
 MSC Classifications: 51M20 - Polyhedra and polytopes; regular figures, division of spaces [See also 51F15] 05C25 - Graphs and abstract algebra (groups, rings, fields, etc.) [See also 20F65] 05C10 - Planar graphs; geometric and topological aspects of graph theory [See also 57M15, 57M25] 05C30 - Enumeration in graph theory 52B70 - Polyhedral manifolds