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Dazhao Tang

Publications and source records attributed to Dazhao Tang.

At least 19 recordsLinked to original sources
arXiv2026

An elementary approach to Sun Kim's general theta function identities

Ramanujan's modular equations of degrees $3$, $5$, $7$, $11$ and $23$ are closely related to certain theta function identities. Warnaar generalized the identities arising from the modular equations of degrees $3$ and $7$ to a general theta function identity. Kim subsequently obtained a further general theta function identity associated with the modular equations of degrees $5$, $11$ and $23$, and established several general theta function identities containing known partition theorems as special cases. In this paper, we present an elementary approach to Kim's general theta function identities, yielding $q$-series proofs of these identities within a common framework.

math.NT↗
arXiv2026

On the occurrence of congruence multiplicities between Ramanujan's theta functions

Recently, the first and third authors initiated a study of arithmetical relationships between Ramanujan's theta functions $\varphi(-q)$ and $\psi(q)$. In this work, we prove eight families of internal congruences modulo arbitrary powers of $3$ and $5$ for infinite series related to the two theta functions. We also show that there exist isomorphisms between the congruence families for $\varphi(-q)$ and $\psi(q)$, which can be realized by the study of congruence multiplicities previously studied by Garvan, Sellers, and the second author. We believe that such equivalences are exclusive, at least on the congruence subgroups $\Gamma_0(6)$ and $\Gamma_0(10)$. In the end, we show how a simple manipulation of function field extensions allows us to predict whether additional isomorphisms to our congruences occur.

math.NT↗
arXiv2026

Identities and transformations for Lambert series and double Lambert series

We establish two identities for Lambert series and double Lambert series, thereby resolving conjectures of Andrews, Dixit, Schultz and Yee (Acta Arith.~181:253--286, 2017), as well as Amdeberhan, Andrews and Ballantine (J Combin Theory Series A 221:106154, 2026). The proofs are based on classical transformations in the theory of infinite series together with a systematic rearrangement of double Lambert series.

math.NT↗
arXiv2025

A conjecture of Nadji, Ahmia and Ram\'{\i}rez on congruences for biregular overpartitions

Let $\overline{B}_{s,t}(n)$ denote the number of overpartitions of $n$ where no part is divisible by $s$ or $t$, with $s$ and $t$ being coprime. By establishing the exact generating functions of a family of arithmetic progressions in $\overline{B}_{4,3}(n)$, we prove that for any $k\geq1$ and $n\geq1$, \begin{align*} \overline{B}_{4,3}\big(2^{k+3}n\big)\equiv0\pmod{2^{3k+5}}. \end{align*} This significantly generalizes a conjectural congruence family posed by Nadji, Ahmia and Ram\'{\i}rez (Ramanujan J. 67 (1):13, 2025) recently. Moreover, we conjecture that there is an infinite family of linear congruence relations modulo high powers of $2$ satisfied by $\overline{B}_{4,3}(n)$.

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arXiv2023

Ramanujan's theta functions and internal congruences modulo arbitrary powers of $3$

In this work, we investigate internal congruences modulo arbitrary powers of $3$ for two functions arising from Ramanujan's classical theta functions $\varphi(q)$ and $\psi(q)$. By letting \begin{align*} \sum_{n\ge 0} ph_3(n) q^n:=\dfrac{\varphi(-q^3)}{\varphi(-q)}\qquad\text{and}\qquad \sum_{n\ge 0} ps_3(n) q^n:=\dfrac{\psi(q^3)}{\psi(q)}, \end{align*} we prove that for any $m\ge 1$ and $n\ge 0$, \begin{align*} ph_3\big(3^{2m-1}n\big)\equiv ph_3\big(3^{2m+1}n\big)\pmod{3^{m+2}}, \end{align*} and \begin{align*} ps_3{\left(3^{2m-1}n+\frac{3^{2m}-1}{4}\right)}\equiv ps_3{\left(3^{2m+1}n+\frac{3^{2m+2}-1}{4}\right)}\pmod{3^{m+2}}, \end{align*} thereby substantially generalizing the previous results of Bharadwaj et al.~and Gireesh et al., respectively.

math.NT↗
arXiv2023

Partitions with parts separated by parity: conjugation, congruences and the mock theta functions

Noting a curious link between Andrews' even-odd crank and the Stanley rank, we adopt a combinatorial approach building on the map of conjugation and continue the study of integer partitions with parts separated by parity. Our motivation is twofold. First off, we derive results for certain restricted partitions with even parts below odd parts. These include a Franklin-type involution proving a parametrized identity that generalizes Andrews' bivariate generating function, and two families of Andrews--Beck type congruences. Secondly, we introduce several new subsets of partitions that are stable (i.e., invariant under conjugation) and explore their connections with three third order mock theta functions $\omega(q)$, $\nu(q)$, and $\psi^{(3)}(q)$, introduced by Ramanujan and Watson.

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arXiv2022

General coefficient-vanishing results associated with theta series

There are a number of sporadic coefficient-vanishing results associated with theta series, which suggest certain underlying patterns. By expanding theta powers as linear combinations of products of theta functions, we present two strategies that will provide a unified treatment. Our approaches rely on studying the behavior of products of two theta series under the action of the huffing operator. For this purpose, some explicit criteria are given. We may use the presented methods to not only verify experimentally discovered coefficient-vanishing results, but also to produce a series of general phenomena.

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arXiv2022

A conjecture of Baruah and Begum on the smallest parts function of restricted overpartitions

In 2017, Andrews, Dixit, Schultz and Yee introduced the function $\overline{\textrm{spt}}_\omega(n)$, which denotes the number of smallest parts in the overpartitions of $n$ in which the smallest part is always overlined and all odd parts are less than twice the smallest part. Recently, Baruah and Begum established several internal congruences and congruences modulo small powers of $5$ for $\overline{\textrm{spt}}_\omega(n)$. Moreover, they conjectured a family of internal congruences modulo any powers of $5$ and two families of congruences modulo any even powers of $5$. In this paper, we confirm three families of congruences due to Baruah and Begum.

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arXiv2020

5-Dissections and sign patterns of Ramanujan's parameter and its companion

In 1998, Michael Hirschhorn discovered 5-dissections of the Rogers--Ramanujan continued fraction $R(q)$ and its reciprocal. In this paper, we obtain the 5-dissections for functions $R(q)R(q^2)^2$ and $R(q)^2/R(q^2)$, which are essentially Ramanujan's parameter and its companion. 5-Dissections of the reciprocals of these two functions are derived as well. These 5-dissections imply that the coefficients in their series expansions have periodic sign patterns with few exceptions.

math.CO↗
arXiv2019

Vanishing coefficients in some $q$-series expansions

Motivated by the recent work of Hirschhorn on vanishing coefficients of the arithmetic progressions in certain $q$-series expansions, we study some variants of these $q$-series and prove some comparable results. For instance, let \begin{align*} (-q,-q^{4};q^{5})_{\infty}^{2}(q^{4},q^{6};q^{10})_{\infty}=\sum_{n=0}^{\infty}a_{1}(n)q^{n}, \end{align*} then \begin{align*} a_{1}(5n+3)=0. \end{align*}

math.CO↗
arXiv2019

A lecture hall theorem for $m$-falling partitions

For an integer $m\ge 2$, a partition $\lambda=(\lambda_1,\lambda_2,\ldots)$ is called $m$-falling, a notion introduced by Keith, if the least nonnegative residues mod $m$ of $\lambda_i$'s form a nonincreasing sequence. We extend a bijection originally due to the third author to deduce a lecture hall theorem for such $m$-falling partitions. A special case of this result gives rise to a finite version of Pak-Postnikov's $(m,c)$-generalization of Euler's theorem. Our work is partially motivated by a recent extension of Euler's theorem for all moduli, due to Keith and Xiong. We note that their result actually can be refined with one more parameter.

math.CO↗
arXiv2018

Several $q$-series related to Ramanujan's theta functions

Quite recently, the first author investigated vanishing coefficients of the arithmetic progressions in several $q$-series expansions. In this paper, we further study the signs of coefficients in two $q$-series expansions and establish some arithmetic relations for several $q$-series expansions by means of Ramanujan's theta functions. We obtain the 5-dissections of these two $q$-series and give combinatorial interpretations for these dissections. Moreover, we obtain four $q$-series identities involving the aforementioned $q$-series, two of which were proved by Kim and Toh via modular forms.

math.CO↗
arXiv2018

Congruences modulo powers of 3 for 2-color partition triples

Let $p_{k,3}(n)$ enumerate the number of 2-color partition triples of $n$ where one of the colors appears only in parts that are multiples of $k$. In this paper, we prove several infinite families of congruences modulo powers of 3 for $p_{k,3}(n)$ with $k=1, 3$, and $9$. For example, for all integers $n\geq0$ and $\alpha\geq1$, we prove that \begin{align*} p_{3,3}\left(3^{\alpha}n+\dfrac{3^{\alpha}+1}{2}\right) &\equiv0\pmod{3^{\alpha+1}} \end{align*} and \begin{align*} p_{3,3}\left(3^{\alpha+1}n+\dfrac{5\times3^{\alpha}+1}{2}\right) &\equiv0\pmod{3^{\alpha+4}}. \end{align*}

math.CO↗
arXiv2018

$(q,t)$-Catalan numbers: gamma expansions, pattern avoidance and the $(-1)$-phenomenon

The aim of this paper is two-fold. We first prove several new interpretations of a kind of $(q,t)$-Catalan numbers along with their corresponding $\gamma$-expansions using pattern avoiding permutations. Secondly, we give a complete characterization of certain $(-1)$-phenomenon for each subset of permutations avoiding a single pattern of length three, and discuss their $q$-analogues utilizing the newly obtained $q$-$\gamma$-expansions, as well as the continued fraction of a quint-variate generating function due to Shin and the fourth author. Moreover, we enumerate the alternating permutations avoiding simultaneously two patterns, namely $(2413,3142)$ and $(1342,2431)$, of length four, and consider such $(-1)$-phenomenon for these two subsets as well.

math.CO↗
arXiv2018

Some inequalities for Garvan's bicrank function of 2-colored partitions

In order to provide a unified combinatorial interpretation of congruences modulo $5$ for 2-colored partition functions, Garvan introduced a bicrank statistic in terms of weighted vector partitions. In this paper, we obtain some inequalities between the bicrank counts $M^{*}(r,m,n)$ for $m=2$, $3$ and $4$ via their asymptotic formulas and some $q$-series techniques. These inequalities are parallel to Andrews and Lewis' results on the rank and crank counts for ordinary partitions.

math.CO↗
arXiv2018

Multi-dimensional $q$-summations and multi-colored partitions

Motivated by Alladi's recent multi-dimensional generalization of Sylvester's classical identity, we provide a simple combinatorial proof of an overpartition analogue, which contains extra parameters tracking the numbers of overlined parts of different colors. This new identity encompasses a handful of classical results as special cases, such as Cauchy's identity, and the product expressions of three classical theta functions studied by Gauss, Jacobi and Ramanujan.

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