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ALGORITHMS FOR LABELING CONSTANT WEIGHT GRAY CODES

Published online by Cambridge University Press:  27 July 2005

INESSA LEVI
Affiliation:
Department of Mathematics, University of Louisville, Louisville, KY 40292, USA e-mail: levi@louisville.edu, mcfadden@louisville.edu, swseif01@louisville.edu
ROBERT B. McFADDEN
Affiliation:
Department of Mathematics, University of Louisville, Louisville, KY 40292, USA e-mail: levi@louisville.edu, mcfadden@louisville.edu, swseif01@louisville.edu
STEVE SEIF
Affiliation:
Department of Mathematics, University of Louisville, Louisville, KY 40292, USA e-mail: levi@louisville.edu, mcfadden@louisville.edu, swseif01@louisville.edu
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Abstract

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Let $n$ and $r$ be positive integers with $1\,{<}\,r\,{<}\,n$, and let $X_n\,{=}\break\{1,2,\ldots,n\}$. An $r$-set $A$ and a partition $\pi$ of $X_n$ are said to be orthogonal if every class of $\pi$ meets $A$ in exactly one element. We prove that if $A_{1},A_{2},\ldots, A_{\binom n r}$ is a list of the distinct $r$-sets of $X_ n$ with $|A_{i}\cap A_{i+1}|\,{=}\,r-1$ for $i=1,2,\ldots, \binom n r$ taken modulo $\binom n r$, then there exists a list of distinct partitions $\pi_{1},\pi_{2},\ldots, \pi_{\binom n r}$ such that $\pi_{i}$ is orthogonal to both $A_{i}$ and $A_{i+1}$. This result states that any constant weight Gray code admits a labeling by distinct orthogonal partitions. Using an algorithm from the literature on Gray codes, we provide a surprisingly efficient algorithm that on input $(n,r)$ outputs an orthogonally labeled constant weight Gray code. We also prove a two-fold Gray enumeration result, presenting an orthogonally labeled constant weight Gray code in which the partition labels form a cycle in the covering graph of the lattice of all partitions of $X_n$. This leads to a conjecture related to the Middle Levels Conjecture. Finally, we provide an application of our results to calculating minimal generating sets of idempotents for finite semigroups.

Keywords

Type
Research Article
Copyright
2005 Glasgow Mathematical Journal Trust