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arXiv · 2311.06229

Absolute retracts of reflexive oriented graphs: the role of the MacNeille completion

Hans-Jürgen Bandelt·Maurice Pouzet·Faouzi Saïdane
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Abstract

We characterize the absolute retracts in the category of reflexive oriented graphs, that is, antisymmetric reflexive graphs, where morphisms between objects preserve arcs (which may be sent to loops). Here we show, by correcting a much earlier attempt at a proof, that a reflexive oriented graph is an absolute retract if and only if it is indeed a retract of some (direct) product of reflexive oriented zigzags (which are concatenations of reflexive oriented paths). Absolute retracts are therefore necessarily acyclic. In contrast to other categories of graphs and ordered sets, not every acyclic oriented graph can be embedded isometrically into some absolute retract. Embedding involves isometry with respect to the zig-zag distances forming a particular "dual quantale", which is a complete lattice of certain sets of words over the alphabet $\{+, -\}$, endowed with an additional monoid operation (viz., compound concatenation of sets of words) and an involution (interchanging $+$ and $- $ and then mirroring words). As reflexive oriented zigzags have MacNeille-closed distances, so do their products and retracts. So, the category of reflexive oriented graphs and its full subcategory of reflexive acyclic graphs do not have enough injectives, as the injective objects coincide with the absolute retracts.

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Hans-Jürgen Bandelt, Maurice Pouzet, Faouzi Saïdane. 2023-11-10. Absolute retracts of reflexive oriented graphs: the role of the MacNeille completion. https://arxiv.org/abs/2311.06229

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