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Percolation in lattice k-neighbor graphs

Published online by Cambridge University Press:  03 June 2025

Benedikt Jahnel*
Affiliation:
Technische Universität Braunschweig
Jonas Köppl*
Affiliation:
Weierstrass Institute for Applied Analysis and Stochastics
Bas Lodewijks*
Affiliation:
Universität Augsburg
András Tóbiás*
Affiliation:
Budapest University of Technology and Economics, and Alfréd Rényi Institute of Mathematics
*
*Postal address: Institut für Mathematische Stochastik, Technische Universität Braunschweig Universitätsplatz 2, 38106 Braunschweig, Germany, and Weierstrass Institute for Applied Analysis and Stochastics, Mohrenstraße 39, 10117 Berlin, Germany. Email: benedikt.jahnel@tu-braunschweig.de
**Postal address: Weierstrass Institute for Applied Analysis and Stochastics, Mohrenstraße 39, 10117 Berlin, Germany. Email: jonas.koeppl@wias-berlin.de
***Postal address: Mathematical Department, Universität Augsburg, Universitätsstraße 2, 86159 Augsburg, Germany. Email: bas.lodewijks@uni-a.de
****Postal address: Department of Computer Science and Information Theory, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary, and HUN-REN Alfréd Rényi Institute of Mathematics, Reáltanoda utca 13-15, H-1053 Budapest, Hungary. Email: tobias@cs.bme.hu

Abstract

We define a random graph obtained by connecting each point of $\mathbb{Z}^d$ independently and uniformly to a fixed number $1 \leq k \leq 2d$ of its nearest neighbors via a directed edge. We call this graph the directed k-neighbor graph. Two natural associated undirected graphs are the undirected and the bidirectional k-neighbor graph, where we connect two vertices by an undirected edge whenever there is a directed edge in the directed k-neighbor graph between the vertices in at least one, respectively precisely two, directions. For these graphs we study the question of percolation, i.e. the existence of an infinite self-avoiding path. Using different kinds of proof techniques for different classes of cases, we show that for $k=1$ even the undirected k-neighbor graph never percolates, while the directed k-neighbor graph percolates whenever $k \geq d+1$, $k \geq 3$, and $d \geq 5$, or $k \geq 4$ and $d=4$. We also show that the undirected 2-neighbor graph percolates for $d=2$, the undirected 3-neighbor graph percolates for $d=3$, and we provide some positive and negative percolation results regarding the bidirectional graph as well. A heuristic argument for high dimensions indicates that this class of models is a natural discrete analogue of the k-nearest-neighbor graphs studied in continuum percolation, and our results support this interpretation.

Type
Original Article
Copyright
© The Author(s), 2025. Published by Cambridge University Press on behalf of Applied Probability Trust

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