An Extension of the Jeu de Taquin

The Knuth transformations on words, the jeu de taquin moves on tableaux, and the Robinson–Schensted–Knuth algorithm produce the same equivalence classes for words. By observing the connections between these three methods we find and prove there exists connections between the Assaf–Knuth transformations, our extension of the jeu de taquin, and p-RSK. We know there exists an algebraic way to expand Macdonald polynomials in terms of the Schur functions. The form of the expansion implies there should be a combinatorial way to find the expansion. Loehr found a Robinson–Schensted–Knuth like algorithm that works in some cases. By finding an extension of jeu de taquin, we will try to expand the number of cases covered

A conjecture about Artin-Tits groups

We conjecture that the word problem of Artin-Tits groups can be solved without introducing trivial factors ss^{-1} or s^{-1}s. Here we make this statement precise and explain how it can be seen as a weak form of hyperbolicity. We prove the conjecture in the case of Artin-Tits groups of type FC, and we discuss various possible approaches for further extensions, in particular a syntactic argument that works at least in the right-angled case

Presentations of higher dimensional Thompson groups

In a previous paper, we defined a higher dimensional analog of Thompson's group V, and proved that it is simple, infinite, finitely generated, and not isomorphic to any of the known Thompson groups. There are other Thompson groups that are infinite, simple and finitely presented. Here we show that the new group is also finitely presented by calculating an explicit finite presentation.Comment: 35 pages, to appear in J. Algebr

The Alternating BWT: An algorithmic perspective

The Burrows-Wheeler Transform (BWT) is a word transformation introduced in 1994 for Data Compression. It has become a fundamental tool for designing self-indexing data structures, with important applications in several areas in science and engineering. The Alternating Burrows-Wheeler Transform (ABWT) is another transformation recently introduced in Gessel et al. (2012) [21] and studied in the field of Combinatorics on Words. It is analogous to the BWT, except that it uses an alternating lexicographical order instead of the usual one. Building on results in Giancarlo et al. (2018) [23], where we have shown that BWT and ABWT are part of a larger class of reversible transformations, here we provide a combinatorial and algorithmic study of the novel transform ABWT. We establish a deep analogy between BWT and ABWT by proving they are the only ones in the above mentioned class to be rank-invertible, a novel notion guaranteeing efficient invertibility. In addition, we show that the backward-search procedure can be efficiently generalized to the ABWT; this result implies that also the ABWT can be used as a basis for efficient compressed full text indices. Finally, we prove that the ABWT can be efficiently computed by using a combination of the Difference Cover suffix sorting algorithm (K\ue4rkk\ue4inen et al., 2006 [28]) with a linear time algorithm for finding the minimal cyclic rotation of a word with respect to the alternating lexicographical order

Density of Ham- and Lee- non-isometric k-ary Words

Isometric k-ary words have been defined referring to the Hamming and the Lee distances. A word is non-isometric if and only if it has a prefix at distance 2 from the suffix of same length; such a prefix is called 2-error overlap. The limit density of isometric binary words based on the Hamming distance has been evaluated by Klavzar and Shpectorov, obtaining that about 8% of all binary words are isometric. In this paper, the issue is addressed for k-ary words and referring to the Hamming and the Lee distances. Actually, the only meaningful case of Lee-isometric k-ary words is when k=4. It is proved that, when the length of words increases, the limit density of quaternary Ham-isometric words is around 17%, while the limit density of quaternary Lee-isometric words is even bigger, it is about 30%. The results are obtained using combinatorial methods and algorithms for counting the number of k-ary isometric words

On the Complexity of BWT-Runs Minimization via Alphabet Reordering

The Burrows-Wheeler Transform (BWT) has been an essential tool in text compression and indexing. First introduced in 1994, it went on to provide the backbone for the first encoding of the classic suffix tree data structure in space close to the entropy-based lower bound. Recently, there has been the development of compact suffix trees in space proportional to "$r$", the number of runs in the BWT, as well as the appearance of $r$ in the time complexity of new algorithms. Unlike other popular measures of compression, the parameter $r$ is sensitive to the lexicographic ordering given to the text's alphabet. Despite several past attempts to exploit this, a provably efficient algorithm for finding, or approximating, an alphabet ordering which minimizes $r$ has been open for years. We present the first set of results on the computational complexity of minimizing BWT-runs via alphabet reordering. We prove that the decision version of this problem is NP-complete and cannot be solved in time $2^{o(\sigma + \sqrt{n})}$ unless the Exponential Time Hypothesis fails, where $\sigma$ is the size of the alphabet and $n$ is the length of the text. We also show that the optimization problem is APX-hard. In doing so, we relate two previously disparate topics: the optimal traveling salesperson path and the number of runs in the BWT of a text, providing a surprising connection between problems on graphs and text compression. Also, by relating recent results in the field of dictionary compression, we illustrate that an arbitrary alphabet ordering provides a $O(\log^2 n)$-approximation. We provide an optimal linear-time algorithm for the problem of finding a run minimizing ordering on a subset of symbols (occurring only once) under ordering constraints, and prove a generalization of this problem to a class of graphs with BWT like properties called Wheeler graphs is NP-complete