Combinatorial Algorithms for Multidimensional Necklaces

A necklace is an equivalence class of words of length n over an alphabet under the cyclic shift (rotation) operation. As a classical object, there have been many algorithmic results for key operations on necklaces, including counting, generating, ranking, and unranking. This paper generalises the concept of necklaces to the multidimensional setting. We define multidimensional necklaces as an equivalence classes over multidimensional words under the multidimensional cyclic shift operation. Alongside this definition, we generalise several problems from the one dimensional setting to the multidimensional setting for multidimensional necklaces with size (n1, n2, . . . , nd) over an alphabet of size q including: providing closed form equations for counting the number of necklaces; an O(n1 · n2 · . . . · nd) time algorithm for transforming some necklace w̃ to the next necklace in the ordering; an O((n1 ·n2 · . . . ·nd)) time algorithm to rank necklaces (determine the number of necklaces smaller than w̃ in the set of necklaces); an O((n1 · n2 · . . . · nd) · log(q)) time algorithm to unrank multidimensional necklace (determine the i necklace in the set of necklaces). Our results on counting, ranking, and unranking are further extended to the fixed content setting, where every necklace has the same Parikh vector, in other words every necklace shares the same number of occurrences of each symbol. Finally, we study the k-centre problem for necklaces both in the single and multidimensional settings. We provide strong approximation algorithms for solving this problem in both the one dimensional and multidimensional settings. ∗Department of Computer Science, Reykjavik University, Iceland. Email: duncana@ru.is †Royal Holloway University of London, UK. Email: argyrios.deligkas@rhul.ac.uk ‡Materials Innovation Factory, Department of Chemistry, University of Liverpool, UK. Email: Vladimir.Gusev@liverpool.ac.uk §Department of Computer Science, University of Liverpool, UK. Email: potapov@liverpool.ac.uk 1 ar X iv :2 10 8. 01 99 0v 2 [ m at h. C O ] 5 N ov 2 02 1

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