English

Biased partitions of $\mathbb{Z}^n$

Combinatorics 2020-09-04 v2

Abstract

Given a function ff on the vertex set of some graph GG, a scenery, let a simple random walk run over the graph and produce a sequence of values. Is it possible to, with high probability, reconstruct the scenery ff from this random sequence? To show this is impossible for some graphs, Gross and Grupel, call a function f:V{0,1}f:V\to\{0,1\} on the vertex set of a graph G=(V,E)G=(V,E) pp-biased if for each vertex vv the fraction of neighbours on which ff is 1 is exactly pp. Clearly, two pp-biased functions are indistinguishable based on their sceneries. Gross and Grupel construct pp-biased functions on the hypercube {0,1}n\{0,1\}^n and ask for what p[0,1]p\in[0,1] there exist pp-biased functions on Zn\mathbb{Z}^n and additionally how many there are. We fully answer this question by giving a complete characterization of these values of pp. We show that pp-biased functions exist for all p=c/2np=c/2n with c{0,,2n}c\in\{0,\dots,2n\} and, in fact, there are uncountably many of them for every c{1,,2n1}c\in\{1,\dots,2n-1\}. To this end, we construct uncountably many partitions of Zn\mathbb{Z}^n into 2n2n parts such that every element of Zn\mathbb{Z}^n has exactly one neighbour in each part. This additionally shows that not all sceneries on Zn\mathbb{Z}^n can be reconstructed from a sequence of values on attained on a simple random walk.

Keywords

Cite

@article{arxiv.1805.05283,
  title  = {Biased partitions of $\mathbb{Z}^n$},
  author = {Peter van Hintum},
  journal= {arXiv preprint arXiv:1805.05283},
  year   = {2020}
}

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10 pages