Abstract
In the recent study of infinite root systems, fractal patterns of ball packings were observed while visualizing roots in affine space. In this paper, we show that the observed fractals are exactly the ball packings described by Boyd and Maxwell. This correspondence is a corollary of a more fundamental result: given a geometric representation of a Coxeter group in a Lorentz space, the set of limit directions of weights equals the set of limit roots. Additionally, we use Coxeter complexes to describe tangency graphs of the corresponding Boyd–Maxwell ball packings. Finally, we enumerate the Coxeter systems that generate Boyd–Maxwell ball packings.
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Notes
By using the term future- or past-directed, we are assuming that the hyperplane h(x)=0 intersects the light cone only at the origin. If this is not the case, one can replace h(x) with any positively weighted sum of the coordinates. The only requirement is that the hyperplane h(x)=1 is transverse to \(\upphi ^+\); see [20, Section 5.2]
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Acknowledgments
The authors are grateful to George Maxwell for his great availability to check the enumeration results. We thank Christian Stump for helpful discussions, and Christophe Hohlweg and Vivien Ripoll for valuable comments on a preliminary version of this article. We also thank the anonymous referee for careful reading and for pointing us to the work of Calabi and Markus.
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J.-P. Labbé is supported by a FQRNT Doctoral scholarship and SFB Transregio “Discretization in Geometry and Dynamics” (TRR 109).
H. Chen is supported by the Deutsche Forschungsgemeinschaft within the Research Training Group “Methods for Discrete Structures” (GRK 1408). An alternative version of this paper appeared in the Ph.D. thesis of the author [8].
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Chen, H., Labbé, JP. Lorentzian Coxeter systems and Boyd–Maxwell ball packings. Geom Dedicata 174, 43–73 (2015). https://doi.org/10.1007/s10711-014-0004-1
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DOI: https://doi.org/10.1007/s10711-014-0004-1