arXiv subjects

Colin Defant

Publications and source records attributed to Colin Defant.

At least 91 records · Page 5Linked to original sources

arXiv2020

Fertility, Strong Fertility, and Postorder Wilf Equivalence

We introduce "fertility Wilf equivalence," "strong fertility Wilf equivalence," and "postorder Wilf equivalence," three variants of Wilf equivalence for permutation classes that formalize some phenomena that have appeared in the study of West's stack-sorting map. We introduce "sliding operators" and show that they induce useful bijections among sets of valid hook configurations. Combining these maps with natural decompositions of valid hook configurations, we give infinitely many examples of fertility, strong fertility, and postorder Wilf equivalences. As a consequence, we obtain infinitely many joint equidistribution results concerning many permutation statistics. In one very special case, we reprove and extensively generalize a result of Bouvel and Guibert. Another case reproves and generalizes a result of the current author. A separate very special case proves and generalizes a conjecture of the current author concerning stack-sorting preimages and the Boolean-Catalan numbers. We end with two open questions.

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arXiv2020

Counting 3-Stack-Sortable Permutations

We prove a "decomposition lemma" that allows us to count preimages of certain sets of permutations under West's stack-sorting map $s$. As a first application, we give a new proof of Zeilberger's formula for the number of 2-stack-sortable permutations in $S_n$. Our proof generalizes, allowing us to find an algebraic equation satisfied by the generating function that counts 2-stack-sortable permutations according to length, number of descents, and number of peaks. The same method yields a recurrence relation for $W_3(n)$, the number of 3-stack-sortable permutations in $S_n$. We compute $W_3(n)$ for $n\le 174$, extending the 13 terms of this sequence that were known before. We also prove the first nontrivial lower bound for $\lim\limits_{n\to\infty}W_3(n)^{1/n}$. Invoking a result of Kremer, we also prove that $\lim\limits_{n\to\infty}W_t(n)^{1/n}\geq(\sqrt{t}+1)^2$ for all $t\geq 1$, which we use to improve a result of Smith. Our computations allow us to disprove a conjecture of Bóna, although we do not yet know for sure which one. We can refine our methods to obtain a recurrence for the number of 3-stack-sortable permutations in $S_n$ with $k$ descents and $p$ peaks. This produces a large amount of evidence supporting a real-rootedness conjecture of Bóna. Using part of the theory of valid hook configurations, we give a new proof of a $γ$-nonnegativity result of Brändén, which in turn implies an older result of Bóna. We then answer a question of the current author by producing a set $A\subseteq S_{11}$ such that $\sum_{σ\in s^{-1}(A)}x^{\text{des}(σ)}$ has nonreal roots. We interpret this as partial evidence against the same real-rootedness conjecture of Bóna that we found evidence supporting. Examining the parities of the numbers $W_3(n)$, we obtain strong evidence against yet another conjecture of Bóna. We end with some conjectures of our own.

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arXiv2019

Fertility Numbers

A nonnegative integer is called a fertility number if it is equal to the number of preimages of a permutation under West's stack-sorting map. We prove structural results concerning permutations, allowing us to deduce information about the set of fertility numbers. In particular, the set of fertility numbers is closed under multiplication and contains every nonnegative integer that is not congruent to $3$ modulo $4$. We show that the lower asymptotic density of the set of fertility numbers is at least $1954/2565\approx 0.7618$. We also exhibit some positive integers that are not fertility numbers and conjecture that there are infinitely many such numbers.

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arXiv2019

Proofs of Conjectures about Pattern-Avoiding Linear Extensions

After fixing a canonical ordering (or labeling) of the elements of a finite poset, one can associate each linear extension of the poset with a permutation. Some recent papers consider specific families of posets and ask how many linear extensions give rise to permutations that avoid certain patterns. We build off of two of these papers. We first consider pattern avoidance in $k$-ary heaps, where we obtain a general result that proves a conjecture of Levin, Pudwell, Riehl, and Sandberg in a special case. We then prove some conjectures that Anderson, Egge, Riehl, Ryan, Steinke, and Vaughan made about pattern-avoiding linear extensions of rectangular posets.

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arXiv2019

Descents in $t$-Sorted Permutations

Let $s$ denote West's stack-sorting map. A permutation is called $t-\textit{sorted}$ if it is of the form $s^t(μ)$ for some permutation $μ$. We prove that the maximum number of descents that a $t$-sorted permutation of length $n$ can have is $\left\lfloor\frac{n-t}{2}\right\rfloor$. When $n$ and $t$ have the same parity and $t\geq 2$, we give a simple characterization of those $t$-sorted permutations in $S_n$ that attain this maximum. In particular, the number of such permutations is $(n-t-1)!!$.

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arXiv2019

Isoperimetry, Stability, and Irredundance in Direct Products

The direct product of graphs $G_1,\ldots,G_n$ is the graph with vertex set $V(G_1)\times\cdots\times V(G_n)$ in which two vertices $(g_1,\ldots,g_n)$ and $(g_1',\ldots,g_n')$ are adjacent if and only if $g_i$ is adjacent to $g_i'$ in $G_i$ for all $i$. Building off of the recent work of Brakensiek, we prove an optimal vertex isoperimetric inequality for direct products of complete multipartite graphs. Applying this inequality, we derive a stability result for independent sets in direct products of balanced complete multipartite graphs, showing that every large independent set must be close to the maximal independent set determined by setting one of the coordinates to be constant. Armed with these isoperimetry and stability results, we prove that the upper irredundance number of a direct product of balanced complete multipartite graphs is equal to its independence number in all but at most $37$ cases. This proves most of a conjecture of Burcroff that arose as a strengthening of a conjecture of the second author and Iyer. We also propose a further strengthening of Burcroff's conjecture.

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arXiv2019

Stack-Sorting Preimages of Permutation Classes

We extend and generalize many of the enumerative results concerning West's stack-sorting map $s$. First, we prove a useful theorem that allows one to efficiently compute $|s^{-1}(π)|$ for any permutation $π$, answering a question of Bousquet-Mélou. We then enumerate permutations in various sets of the form $s^{-1}(\text{Av}(τ^{(1)},\ldots,τ^{(r)}))$, where $\text{Av}(τ^{(1)},\ldots,τ^{(r)})$ is the set of permutations avoiding the patterns $τ^{(1)},\ldots,τ^{(r)}$. These preimage sets often turn out to be permutation classes themselves, so the current paper represents a new approach, based on the theory of valid hook configurations, for solving classical enumerative problems. In one case, we solve a problem previously posed by Bruner. We are often able to refine our counts by enumerating these permutations according to their number of descents or peaks. Our investigation not only provides several new combinatorial interpretations and identities involving known sequences, but also paves the way for several new enumerative problems.

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arXiv2019

On Anti-Powers in Aperiodic Recurrent Words

Fici, Restivo, Silva, and Zamboni define a $\textit{$k$-anti-power}$ to be a concatenation of $k$ consecutive words that are pairwise distinct and have the same length. They ask for the maximum $k$ such that every aperiodic recurrent word must contain a $k$-anti-power, and they prove that this maximum must be 3, 4, or 5. We resolve this question by demonstrating that the maximum is 5. We also conjecture that if $W$ is a reasonably nice aperiodic morphic word, then there is some constant $C = C(W)$ such that for all $i,k\geq 1$, $W$ contains a $k$-anti-power with blocks of length at most $Ck$ beginning at its $i^\text{th}$ position. We settle this conjecture for binary words that are generated by a uniform morphism, characterizing the small exceptional set of words for which such a constant cannot be found. This generalizes recent results of the second author, Gaetz, and Narayanan that have been proven for the Thue-Morse word, which also show that such a linear bound is the best one can hope for in general.

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arXiv2018

Stack-sorting for Words

We introduce operators $\mathsf{hare}$ and $\mathsf{tortoise}$, which act on words as natural generalizations of West's stack-sorting map. We show that the heuristically slower algorithm $\mathsf{tortoise}$ can sort words arbitrarily faster than its counterpart $\mathsf{hare}$. We then generalize the combinatorial objects known as valid hook configurations in order to find a method for computing the number of preimages of any word under these two operators. We relate the question of determining which words are sortable by $\mathsf{hare}$ and $\mathsf{tortoise}$ to more classical problems in pattern avoidance, and we derive a recurrence for the number of words with a fixed number of copies of each letter (permutations of a multiset) that are sortable by each map. In particular, we use generating trees to prove that the $\ell$-uniform words on the alphabet $[n]$ that avoid the patterns $231$ and $221$ are counted by the $(\ell+1)$-Catalan number $\frac{1}{\ell n+1}{(\ell+1)n\choose n}$. We conclude with several open problems and conjectures.

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arXiv2018

On the genus of a quotient of a numerical semigroup

We find a relation between the genus of a quotient of a numerical semigroup $S$ and the genus of $S$ itself. We use this identity to compute the genus of a quotient of $S$ when $S$ has embedding dimension $2$. We also exhibit identities relating the Frobenius numbers and the genus of quotients of numerical semigroups that are generated by certain types of arithmetic progressions.

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arXiv2018

Enumerating Cliques in Direct Product Graphs

The unitary Cayley graph of $\mathbb Z/n\mathbb Z$, denoted $G_{\mathbb Z/n\mathbb Z}$, is the graph with vertices $0,1,\ldots,$ $n-1$ in which two vertices are adjacent if and only if their difference is relatively prime to $n$. These graphs are central to the study of graph representations modulo integers, which were originally introduced by Erdős and Evans. We give a brief account of some results concerning these beautiful graphs and provide a short proof of a simple formula for the number of cliques of any order $m$ in the unitary Cayley graph $G_{\mathbb Z/n\mathbb Z}$. This formula involves an exciting class of arithmetic functions known as Schemmel totient functions, which we also briefly discuss. More generally, the proof yields a formula for the number of cliques of order $m$ in a direct product of balanced complete multipartite graphs.

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arXiv2018

Domination and Upper Domination of Direct Product Graphs

The unitary Cayley graph of $\mathbb{Z} /n \mathbb{Z}$, denoted $X_{\mathbb{Z} / n \mathbb{Z}}$, has vertices $0,1, \dots, n-1$ with $x$ adjacent to $y$ if $x-y$ is relatively prime to $n$. We present results on the tightness of the known inequality $γ(X_{\mathbb{Z} / n \mathbb{Z}})\leq γ_t(X_{\mathbb{Z} / n \mathbb{Z}})\leq g(n)$, where $γ$ and $γ_t$ denote the domination number and total domination number, respectively, and $g$ is the arithmetic function known as Jacobsthal's function. In particular, we construct integers $n$ with arbitrarily many distinct prime factors such that $γ(X_{\mathbb{Z} / n \mathbb{Z}})\leqγ_t(X_{\mathbb{Z} / n \mathbb{Z}})\leq g(n)-1$. Extending work of Mekiš, we give lower bounds for the domination numbers of direct products of complete graphs. We also present a simple conjecture for the exact values of the upper domination numbers of direct products of balanced, complete multipartite graphs and prove the conjecture in certain cases. We end with some open problems.

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arXiv2018

Ranges of Unitary Divisor Functions

For any real $t$, the unitary divisor function $σ_t^*$ is the multiplicative arithmetic function defined by $σ_t^*(p^α)=1+p^{αt}$ for all primes $p$ and positive integers $α$. Let $\overline{σ_t^*(\mathbb N)}$ denote the topological closure of the range $σ_t^*$. We calculate an explicit constant $η^*\approx 1.9742550$ and show that $\overline{σ_{-r}^*(\mathbb N)}$ is connected if and only if $r\in(0,η^*]$. We end with an open problem.

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arXiv2018

Preimages under the Stack-Sorting Algorithm

We use a method for determining the number of preimages of any permutation under the stack-sorting map in order to obtain recursive upper bounds for the numbers $W_t(n)$ and $W_t(n,k)$ of $t$-stack sortable permutations of length $n$ and $t$-stack sortable permutations of length $n$ with exactly $k$ descents. From these bounds, we are able to significantly improve the best known upper bounds for $\displaystyle{\lim_{n\to\infty}\sqrt[n]{W_t(n)}}$ when $t=3$ and $t=4$.

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arXiv2018

Postorder Preimages

Given a set $Y$ of decreasing plane trees and a permutation $π$, how many trees in $Y$ have $π$ as their postorder? Using combinatorial and geometric constructions, we provide a method for answering this question for certain sets $Y$ and all permutations $π$. We then provide applications of our results to the study of the deterministic stack-sorting algorithm.

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arXiv2018

Flexible Toggles and Symmetric Invertible Asynchronous Elementary Cellular Automata

A sequential dynamical system (SDS) consists of a graph $G$ with vertices $v_1,v_2,\ldots,v_n$, a state set $A$, a collection of "vertex functions" $\{f_{v_i}\}_{i=1}^n$, and a permutation $π\in S_n$ that specifies how to compose these functions to yield the SDS map $[G,\{f_{v_i}\}_{i=1}^n,π]\colon A^n\to A^n$. In this paper, we study symmetric invertible SDS defined over the cycle graph $C_n$ using the set of states $\mathbb F_2$. These are, in other words, asynchronous elementary cellular automata (ECA) defined using ECA rules 150 and 105. Each of these SDS defines a group action on the set $\mathbb F_2^n$ of $n$-bit binary vectors. Because the SDS maps are products of involutions, this relates to \emph{generalized toggle groups}, which Striker recently defined. In this paper, we further generalize the notion of a generalized toggle group to that of a \emph{flexible toggle group}; the SDS maps we consider are examples of Coxeter elements of flexible toggle groups. Our main result is the complete classification of the dynamics of symmetric invertible SDS defined over cycle graphs using the set of states $\mathbb F_2$ and the identity update order $π=123\cdots n$. More precisely, if $T$ denotes the SDS map of such an SDS, then we obtain an explicit formula for $|\text{Per}_r(T)|$, the number of periodic points of $T$ of period $r$, for every positive integer $r$. It turns out that if we fix $r$ and vary $n$ and $T$, then $|\text{Per}_r(T)|$ only takes at most three nonzero values.

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arXiv2018

Connected Components of Complex Divisor Functions

For any complex number $c$, define the divisor function $σ_c\colon\mathbb N\to\mathbb C$ by $\displaystyleσ_c(n)=\sum_{d\mid n}d^c$. Let $\overline{σ_c(\mathbb N)}$ denote the topological closure of the range of $σ_c$. Extending previous work of the current author and Sanna, we prove that $\overline{σ_c(\mathbb N)}$ has nonempty interior and has finitely many connected components if $\Re(c)\leq 0$ and $c\neq 0$. We end with some open problems.

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arXiv2018

Complex Divisor Functions

For any complex number $c$, let $σ_c\colon\mathbb N\rightarrow\mathbb C$ denote the divisor function defined by $σ_c(n)=\displaystyle{\sum_{d|n}d^c}$ for all $n\in\mathbb N$, and define $R(c)=\{σ_c(n)\in\mathbb C\colon n\in\mathbb N\}$ to be the range of $σ_c$. We study the basic topological properties of the sets $R(c)$. In particular, we determine the complex numbers $c$ for which $R(c)$ is bounded and determine the isolated points of the sets $R(c)$. In the third section, we find those values of $c$ for which $R(c)$ is dense in $\mathbb C$. We also prove some results and pose several open problems about the closures of the sets $R(c)$ when these sets are bounded.

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