arXiv subjects

William Y. C. Chen

Publications and source records attributed to William Y. C. Chen.

At least 37 records · Page 2Linked to original sources

arXiv2017

Stanley's Lemma and Multiple Theta Functions

We present an algorithmic approach to the verification of identities on multiple theta functions in the form of products of theta functions $[(-1)^δa_1^{α_1}a_2^{α_2}\cdots a_r^{α_r}q^{s}; q^{t}]_\infty$, where $α_i$ are integers, $δ=0$ or $1$, $s\in \mathbb{Q}$, $t\in \mathbb{Q}^{+}$, and the exponent vectors $(α_1,α_2,\ldots,α_r)$ are linearly independent over $\mathbb{Q}$. For an identity on such multiple theta functions, we provide an algorithmic approach for computing a system of contiguous relations satisfied by all the involved multiple theta functions. Using Stanley's Lemma on the fundamental parallelepiped, we show that a multiple theta function can be determined by a finite number of its coefficients. Thus such an identity can be reduced to a finite number of simpler relations. Many classical multiple theta function identities fall into this framework, including Riemann's addition formula and the extended Riemann identity.

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arXiv2017

Higher Order Turán Inequalities for the Partition Function

The Turán inequalities and the higher order Turán inequalities arise in the study of Maclaurin coefficients of an entire function in the Laguerre-Pólya class. A real sequence $\{a_{n}\}$ is said to satisfy the Turán inequalities if for $n\geq 1$, $a_n^2-a_{n-1}a_{n+1}\geq 0$. It is said to satisfy the higher order Turán inequalities if for $n\geq 1$, $4(a_{n}^2-a_{n-1}a_{n+1})(a_{n+1}^2-a_{n}a_{n+2})-(a_{n}a_{n+1}-a_{n-1}a_{n+2})^2\geq 0$. A sequence satisfying the Turán inequalities is also called log-concave. For the partition function $p(n)$, DeSalvo and Pak showed that for $n>25$, the sequence $\{ p(n)\}_{n> 25}$ is log-concave, that is, $p(n)^2-p(n-1)p(n+1)>0$ for $n> 25$. It was conjectured by Chen that $p(n)$ satisfies the higher order Turán inequalities for $n\geq 95$. In this paper, we prove this conjecture by using the Hardy-Ramanujan-Rademacher formula to derive an upper bound and a lower bound for $p(n+1)p(n-1)/p(n)^2$. Consequently, for $n\geq 95$, the Jensen polynomials $g_{3,n-1}(x)=p(n-1)+3p(n)x+3p(n+1)x^2+p(n+2)x^3$ have only real zeros. We conjecture that for any positive integer $m\geq 4$ there exists an integer $N(m)$ such that for $n\geq N(m) $, the polynomials $\sum_{k=0}^m {m\choose k}p(n+k)x^k$ have only real zeros. This conjecture was independently posed by Ono.

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arXiv2017

Nearly Equal Distributions of the Rank and the Crank of Partitions

Let $N(\leq m,n)$ denote the number of partitions of $n$ with rank not greater than $m$, and let $M(\leq m,n)$ denote the number of partitions of $n$ with crank not greater than $m$. Bringmann and Mahlburg observed that $N(\leq m,n)\leq M(\leq m,n)\leq N(\leq m+1,n)$ for $m<0$ and $1\leq n\leq 100$. They also pointed out that these inequalities can be restated as the existence of a re-ordering $τ_n$ on the set of partitions of $n$ such that $|\text{crank}(λ)|-|\text{rank}(τ_n(λ))|=0$ or $1$ for all partitions $λ$ of $n$, that is, the rank and the crank are nearly equal distributions over partitions of $n$. In the study of the spt-function, Andrews, Dyson and Rhoades proposed a conjecture on the unimodality of the spt-crank, and they showed that this conjecture is equivalent to the inequality $N(\leq m,n)\leq M(\leq m,n)$ for $m<0$ and $n\geq 1$. We proved this conjecture by combiantorial arguments. In this paper, we prove the inequality $N(\leq m,n)\leq M(\leq m,n)$ for $m<0$ and $n\geq 1$. Furthermore, we define a re-ordering $τ_n$ of the partitions $λ$ of $n$ and show that this re-ordering $τ_n$ leads to the nearly equal distribution of the rank and the crank. Using the re-ordering $τ_n$, we give a new combinatorial interpretation of the function ospt$(n)$ defined by Andrews, Chan and Kim, which immediately leads to an upper bound for $ospt(n)$ due to Chan and Mao.

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arXiv2015

The Log-Behavior of $\sqrt[n]{p(n)}$ and $\sqrt[n]{p(n)/n}$

Let $p(n)$ denote the partition function. Desalvo and Pak proved the log-concavity of $p(n)$ for $n>25$ and the inequality $\frac{p(n-1)}{p(n)}\left(1+\frac{1}{n}\right)>\frac{p(n)}{p(n+1)}$ for $n>1$. Let $r(n)=\sqrt[n]{p(n)/n}$ and $Δ$ be the difference operator respect to $n$. Desalvo and Pak pointed out that their approach to proving the log-concavity of $p(n)$ may be employed to prove a conjecture of Sun on the log-convexity of $\{r(n)\}_{n\geq 61}$, as long as one finds an appropriate estimate of $Δ^2 \log r(n-1)$. In this paper, we obtain a lower bound for $Δ^2\log r(n-1)$, leading to a proof of this conjecture. From the log-convexity of $\{r(n)\}_{n\geq61}$ and $\{\sqrt[n]{n}\}_{n\geq4}$, we are led to a proof of another conjecture of Sun on the log-convexity of $\{\sqrt[n]{p(n)}\}_{n\geq27}$. Furthermore, we show that $\lim\limits_{n \rightarrow +\infty}n^{\frac{5}{2}}Δ^2\log\sqrt[n]{p(n)}=3π/\sqrt{24}$. Finally, by finding an upper bound of $Δ^2 \log\sqrt[n-1]{p(n-1)}$, we prove an inequality on the ratio $\frac{\sqrt[n-1]{p(n-1)}}{\sqrt[n]{p(n)}}$ analogous to the above inequality on the ratio $\frac{p(n-1)}{p(n)}$.

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arXiv2015

Automated Discovery and Proof of Congruence Theorems for Partial Sums of Combinatorial Sequences

Many combinatorial sequences (for example, the Catalan and Motzkin numbers) may be expressed as the constant term of $P(x)^k Q(x)$, for some Laurent polynomials $P(x)$ and $Q(x)$ in the variable $x$ with integer coefficients. Denoting such a sequence by $a_k$, we obtain a general formula that determines the congruence class, modulo $p$, of the indefinite sum $\sum_{k=0}^{rp -1} a_k$, for {\it any} prime $p$, and any positive integer $r$, as a linear combination of sequences that satisfy linear recurrence (alias difference) equations with constant coefficients. This enables us (or rather, our computers) to automatically discover and prove congruence theorems for such partial sums. Moreover, we show that in many cases, the set of the residues is finite, regardless of the prime $p$.

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arXiv2014

Context-free Grammars for Permutations and Increasing Trees

In this paper, we introduce the notion of a grammatical labeling to describe a recursive process of generating combinatorial objects based on a context-free grammar. For example, by labeling the ascents and descents of a Stirling permutation, we obtain a grammar for the second-order Eulerian polynomials. By using the grammar for $0$-$1$-$2$ increasing trees given by Dumont, we obtain a grammatical derivation of the generating function of the André polynomials obtained by Foata and Schützenberger, without solving a differential equation. We also find a grammar for the number $T(n,k)$ of permutations of $[n]=\{1,2,\ldots, n\}$ with $k$ exterior peaks, which was independently discovered by Ma. We demonstrate that Gessel's formula for the generating function of $T(n,k)$ can be deduced from this grammar. Moreover, by using grammars we show that the number of the permutations of $[n]$ with $k$ exterior peaks equals the number of increasing trees on $[n]$ with $2k+1$ vertices of even degree. A combinatorial proof of this fact is also presented.

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arXiv2014

Ramanujan-type Congruences for Overpartitions Modulo 16

Let $\overline{p}(n)$ denote the number of overpartitions of $n$. Recently, Fortin-Jacob-Mathieu and Hirschhorn-Sellers independently obtained 2-, 3- and 4-dissections of the generating function for $\overline{p}(n)$ and derived a number of congruences for $\overline{p}(n)$ modulo $4$, $8$ and $64$ including $\overline{p}(5n+2)\equiv 0 \pmod{4}$, $\overline{p}(4n+3)\equiv 0 \pmod{8}$ and $\overline{p}(8n+7)\equiv 0 \pmod{64}$. By employing dissection techniques, Yao and Xia obtained congruences for $\overline{p}(n)$ modulo $8, 16$ and $32$, such as $\overline{p}(48n+26) \equiv 0 \pmod{8}$, $\overline{p}(24n+17)\equiv 0 \pmod{16}$ and $\overline{p}(72n+69)\equiv 0 \pmod{32}$. In this paper, we give a 16-dissection of the generating function for $\overline{p}(n)$ modulo 16 and we show that $\overline{p}(16n+14)\equiv0\pmod{16}$ for $n\ge 0$. Moreover, by using the $2$-adic expansion of the generating function of $\overline{p}(n)$ due to Mahlburg, we obtain that $\overline{p}(\ell^2n+r\ell)\equiv0\pmod{16}$, where $n\ge 0$, $\ell \equiv -1\pmod{8}$ is an odd prime and $r$ is a positive integer with $\ell \nmid r$. In particular, for $\ell=7$, we get $\overline{p}(49n+7)\equiv0\pmod{16}$ and $\overline{p}(49n+14)\equiv0\pmod{16}$ for $n\geq 0$. We also find four congruence relations: $\overline{p}(4n)\equiv(-1)^n\overline{p}(n) \pmod{16}$ for $n\ge 0$, $\overline{p}(4n)\equiv(-1)^n\overline{p}(n)\pmod{32}$ for $n$ being not a square of an odd positive integer, $\overline{p}(4n)\equiv(-1)^n\overline{p}(n)\pmod{64}$ for $n\not\equiv 1,2,5\pmod{8}$ and $\overline{p}(4n)\equiv(-1)^n\overline{p}(n)\pmod{128}$ for $n\equiv 0\pmod{4}$.

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arXiv2014

Finite Differences of the Logarithm of the Partition Function

Let $p(n)$ denote the partition function. DeSalvo and Pak proved that $\frac{p(n-1)}{p(n)}\left(1+\frac{1}{n}\right)> \frac{p(n)}{p(n+1)}$ for $n\geq 2$, as conjectured by Chen. Moreover, they conjectured that a sharper inequality $\frac{p(n-1)}{p(n)}\left( 1+\fracπ{\sqrt{24}n^{3/2}}\right) > \frac{p(n)}{p(n+1)}$ holds for $n\geq 45$. In this paper, we prove the conjecture of Desalvo and Pak by giving an upper bound for $-Δ^{2} \log p(n-1)$, where $Δ$ is the difference operator with respect to $n$. We also show that for given $r\geq 1$ and sufficiently large $n$, $(-1)^{r-1}Δ^{r} \log p(n)>0$. This is analogous to the positivity of finite differences of the partition function. It was conjectured by Good and proved by Gupta that for given $r\geq 1$, $Δ^{r} p(n)>0$ for sufficiently large $n$.

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arXiv2014

Ramanujan-type Congruences for Overpartitions Modulo 5

Let $\overline{p}(n)$ denote the number of overpartitions of $n$. Hirschhorn and Sellers showed that $\overline{p}(4n+3)\equiv 0 \pmod{8}$ for $n\geq 0$. They also conjectured that $\overline{p}(40n+35)\equiv 0 \pmod{40}$ for $n\geq 0$. Chen and Xia proved this conjecture by using the $(p,k)$-parametrization of theta functions given by Alaca, Alaca and Williams. In this paper, we show that $\overline{p}(5n)\equiv (-1)^{n}\overline{p}(4\cdot 5n) \pmod{5}$ for $n \geq 0$ and $\overline{p}(n)\equiv (-1)^{n}\overline{p}(4n)\pmod{8}$ for $n \geq 0$ by using the relation of the generating function of $\overline{p}(5n)$ modulo $5$ found by Treneer and the $2$-adic expansion of the generating function of $\overline{p}(n)$ due to Mahlburg. As a consequence, we deduce that $\overline{p}(4^k(40n+35))\equiv 0 \pmod{40}$ for $n,k\geq 0$. Furthermore, applying the Hecke operator on $ϕ(q)^3$ and the fact that $ϕ(q)^3$ is a Hecke eigenform, we obtain an infinite family of congrences $\overline{p}(4^k \cdot5\ell^2n)\equiv 0 \pmod{5}$, where $k\ge 0$ and $\ell$ is a prime such that $\ell\equiv3 \pmod{5}$ and $\left(\frac{-n}{\ell}\right)=-1$. Moreover, we show that $\overline{p}(5^{2}n)\equiv \overline{p}(5^{4}n) \pmod{5}$ for $n \ge 0$. So we are led to the congruences $\overline{p}\big(4^k5^{2i+3}(5n\pm1)\big)\equiv 0 \pmod{5}$ for $n, k, i\ge 0$. In this way, we obtain various Ramanujan-type congruences for $\overline{p}(n)$ modulo $5$ such as $\overline{p}(45(3n+1))\equiv 0 \pmod{5}$ and $\overline{p}(125(5n\pm 1))\equiv 0 \pmod{5}$ for $n\geq 0$.

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arXiv2014

Zigzag Stacks and m-Regular Linear Stacks

The contact map of a protein fold is a graph that represents the patterns of contacts in the fold. It is known that the contact map can be decomposed into stacks and queues. RNA secondary structures are special stacks in which the degree of each vertex is at most one and each arc has length at least two. Waterman and Smith derived a formula for the number of RNA secondary structures of length $n$ with exactly $k$ arcs. Höner zu Siederdissen et al. developed a folding algorithm for extended RNA secondary structures in which each vertex has maximum degree two. An equation for the generating function of extended RNA secondary structures was obtained by Müller and Nebel by using a context-free grammar approach, which leads to an asymptotic formula. In this paper, we consider $m$-regular linear stacks, where each arc has length at least $m$ and the degree of each vertex is bounded by two. Extended RNA secondary structures are exactly $2$-regular linear stacks. For any $m\geq 2$, we obtain an equation for the generating function of the $m$-regular linear stacks. For given $m$, we can deduce a recurrence relation and an asymptotic formula for the number of $m$-regular linear stacks on $n$ vertices. To establish the equation, we use the reduction operation of Chen, Deng and Du to transform an $m$-regular linear stack to an $m$-reduced zigzag (or alternating) stack. Then we find an equation for $m$-reduced zigzag stacks leading to an equation for $m$-regular linear stacks.

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arXiv2014

Vacillating Hecke Tableaux and Linked Partitions

We introduce the structure of vacillating Hecke tableaux, and establish a one-to-one correspondence between vacillating Hecke tableaux and linked partitions by using the Hecke insertion algorithm developed by Buch, Kresch, Shimozono, Tamvakis and Yong. Linked partitions arise in free probability theory. Motivated by the Hecke insertion algorithm, we define a Hecke diagram as a Young diagram possibly with a marked corner. A vacillating Hecke tableau is defined as a sequence of Hecke diagrams subject to certain addition and deletion of rook strips. The notion of a rook strip was introduced by Buch in the study of the Littlewood-Richardson rule for stable Grothendieck polynomials. A rook strip is a skew Young diagram with at most one square in each row and column. We show that the crossing number and the nesting number of a linked partition can be determined by the maximal number of rows and the maximal number of columns of the diagrams in the corresponding vacillating Hecke tableau. The proof relies on a theorem due to Thomas and Yong concerning the lengths of the longest strictly increasing and the longest strictly decreasing subsequences in a word. This implies that the crossing number and the nesting number have a symmetric joint distribution over linked partitions, confirming a conjecture of de Mier. We also prove a conjecture of Kim which states that the crossing number and the nesting number have a symmetric joint distribution over the front representations of partitions.

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arXiv2014

Average Size of a Self-conjugate (s, t)-Core Partition

Armstrong, Hanusa and Jones conjectured that if $s,t$ are coprime integers, then the average size of an $(s,t)$-core partition and the average size of a self-conjugate $(s,t)$-core partition are both equal to $\frac{(s+t+1)(s-1)(t-1)}{24}$. Stanley and Zanello showed that the average size of an $(s,s+1)$-core partition equals $\binom{s+1}{3}/2$. Based on a bijection of Ford, Mai and Sze between self-conjugate $(s,t)$-core partitions and lattice paths in $\lfloor \frac{s}{2} \rfloor\times \lfloor \frac{t}{2}\rfloor$ rectangle, we obtain the average size of a self-conjugate $(s,t)$-core partition as conjectured by Armstrong, Hanusa and Jones.

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arXiv2014

Automorphisms of the cycle prefix digraph

Cycle prefix digraphs have been proposed as an efficient model of symmetric interconnection networks for parallel architecture. It has been discovered that the cycle prefix networks have many attractive communication properties. In this paper, we determine the automorphism group of the cycle prefix digraphs. We show that the automorphism group of a cycle prefix digraph is isomorphic to the symmetric group on its underlying alphabet. Our method can be applied to other classes of graphs built on alphabets including the hypercube, the Kautz graph,and the de Bruijn graph.

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arXiv2013

A Class of Kazhdan-Lusztig R-Polynomials and q-Fibonacci Numbers

Let $S_n$ denote the symmetric group on $\{1,2,\ldots,n\}$. For two permutations $u, v\in S_n$ such that $u\leq v$ in the Bruhat order, let $R_{u,v}(q)$ and $\R_{u,v}(q)$ denote the Kazhdan-Lusztig $R$-polynomial and $\R$-polynomial, respectively. Let $v_n=34\cdots n\, 12$, and let $σ$ be a permutation such that $σ\leq v_n$. We obtain a formula for the $\R$-polynomials $\R_{σ,v_n}(q)$ in terms of the $q$-Fibonacci numbers depending on a parameter determined by the reduced expression of $σ$. When $σ$ is the identity $e$, this reduces to a formula obtained by Pagliacci. In another direction, we obtain a formula for the $\R$-polynomial $\R_{e,\,v_{n,i}}(q)$, where $v_{n,i} = 3 4\cdots i\,n\, (i+1)\cdots (n-1)\, 12$. In a more general context, we conjecture that for any two permutations $σ,τ\in S_n$ such that $σ\leq τ\leq v_n$, the $\R$-polynomial $\R_{σ,τ}(q)$ can be expressed as a product of $q$-Fibonacci numbers multiplied by a power of $q$.

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arXiv2013

k-Marked Dyson Symbols and Congruences for Moments of Cranks

By introducing $k$-marked Durfee symbols, Andrews found a combinatorial interpretation of $2k$-th symmetrized moment $η_{2k}(n)$ of ranks of partitions of $n$. Recently, Garvan introduced the $2k$-th symmetrized moment $μ_{2k}(n)$ of cranks of partitions of $n$ in the study of the higher-order spt-function $spt_k(n)$. In this paper, we give a combinatorial interpretation of $μ_{2k}(n)$. We introduce $k$-marked Dyson symbols based on a representation of ordinary partitions given by Dyson, and we show that $μ_{2k}(n)$ equals the number of $(k+1)$-marked Dyson symbols of $n$. We then introduce the full crank of a $k$-marked Dyson symbol and show that there exist an infinite family of congruences for the full crank function of $k$-marked Dyson symbols which implies that for fixed prime $p\geq 5$ and positive integers $r$ and $k\leq (p-1)/2$, there exist infinitely many non-nested arithmetic progressions $An+B$ such that $μ_{2k}(An+B)\equiv 0\pmod{p^r}$.

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arXiv2013

On the Positive Moments of Ranks of Partitions

By introducing $k$-marked Durfee symbols, Andrews found a combinatorial interpretation of $2k$-th symmetrized moment $η_{2k}(n)$ of ranks of partitions of $n$ in terms of $(k+1)$-marked Durfee symbols of $n$. In this paper, we consider the $k$-th symmetrized positive moment $\barη_k(n)$ of ranks of partitions of $n$ which is defined as the truncated sum over positive ranks of partitions of $n$. As combintorial interpretations of $\barη_{2k}(n)$ and $\barη_{2k-1}(n)$, we show that for fixed $k$ and $i$ with $1\leq i\leq k+1$, $\barη_{2k-1}(n)$ equals the number of $(k+1)$-marked Durfee symbols of $n$ with the $i$-th rank being zero and $\barη_{2k}(n)$ equals the number of $(k+1)$-marked Durfee symbols of $n$ with the $i$-th rank being positive. The interpretations of $\barη_{2k-1}(n)$ and $\barη_{2k}(n)$ also imply the interpretation of $η_{2k}(n)$ given by Andrews since $η_{2k}(n)$ equals $\barη_{2k-1}(n)$ plus twice of $\barη_{2k}(n)$. Moreover, we obtain the generating functions of $\barη_{2k}(n)$ and $\barη_{2k-1}(n)$.

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arXiv2013

s-Inversion Sequences and P-Partitions of Type B

Given a sequence $s=(s_1,s_2,\ldots)$ of positive integers, the inversion sequences with respect to $s$, or $s$-inversion sequences, were introduced by Savage and Schuster in their study of lecture hall polytopes. A sequence $(e_1,e_2,\ldots,e_n)$ of nonnegative integers is called an $s$-inversion sequence of length $n$ if $0\leq e_i < s_i$ for $1\leq i\leq n$. Let I(n) be the set of $s$-inversion sequences of length $n$ for $s=(1,4,3,8,5,12,\ldots)$, that is, $s_{2i}=4i$ and $s_{2i-1}=2i-1$ for $i\geq1$, and let $P_n$ be the set of signed permutations on $\{1^2,2^2,\ldots,n^2\}$. Savage and Visontai conjectured that when $n=2k$, the ascent number over $I_n$ is equidistributed with the descent number over $P_k$. For a positive integer $n$, we use type $B$ $P$-partitions to give a characterization of signed permutations over which the descent number is equidistributed with the ascent number over $I_n$. When $n$ is even, this confirms the conjecture of Savage and Visontai. Moreover, let $I'_n$ be the set of $s$-inversion sequences of length $n$ for $s=(2,2,6,4,10,6,\ldots)$, that is, $s_{2i}=2i$ and $s_{2i-1}=4i-2$ for $i\geq1$. We find a set of signed permutations over which the descent number is equidistributed with the ascent number over $I'_n$.

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arXiv2013

Zeta Functions and the Log-behavior of Combinatorial Sequences

In this paper, we use the Riemann zeta function $ζ(x)$ and the Bessel zeta function $ζ_μ(x)$ to study the log-behavior of combinatorial sequences. We prove that $ζ(x)$ is log-convex for $x>1$. As a consequence, we deduce that the sequence $\{|B_{2n}|/(2n)!\}_{n\geq 1}$ is log-convex, where $B_n$ is the $n$-th Bernoulli number. We introduce the function $θ(x)=(2ζ(x)Γ(x+1))^{\frac{1}{x}}$, where $Γ(x)$ is the gamma function, and we show that $\log θ(x)$ is strictly increasing for $x\geq 6$. This confirms a conjecture of Sun stating that the sequence $\{\sqrt[n] {|B_{2n}}|\}_{n\geq 1}$ is strictly increasing. Amdeberhan, Moll and Vignat defined the numbers $a_n(μ)=2^{2n+1}(n+1)!(μ+1)_nζ_μ(2n)$ and conjectured that the sequence $\{a_n(μ)\}_{n\geq 1}$ is log-convex for $μ=0$ and $μ=1$. By proving that $ζ_μ(x)$ is log-convex for $x>1$ and $μ>-1$, we show that the sequence $\{a_n(μ)\}_{n\geq 1}$ is log-convex for any $μ>-1$. We introduce another function $θ_μ(x)$ involving $ζ_μ(x)$ and the gamma function $Γ(x)$ and we show that $\log θ_μ(x)$ is strictly increasing for $x>8e(μ+2)^2$. This implies that $\sqrt[n]{a_n(μ)}<\sqrt[n+1]{a_{n+1}(μ)}$ for $n> 4e(μ+2)^2$. Based on Dobinski's formula, we prove that $\sqrt[n]{B_n}<\sqrt[n+1]{B_{n+1}}$ for $n\geq 1$, where $B_n$ is the $n$-th Bell number. This confirms another conjecture of Sun. We also establish a connection between the increasing property of $\{\sqrt[n]{B_n}\}_{n\geq 1}$ and Hölder's inequality in probability theory.

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