arXiv subjects

Zhi-Wei Sun

Publications and source records attributed to Zhi-Wei Sun.

At least 127 records · Page 7Linked to original sources

arXiv2017

Telescoping method and congruences for double sums

In recent years, Z.-W. Sun proposed several sophisticated conjectures on congruences for finite sums with terms involving combinatorial sequences such as central trinomial coefficients, Domb numbers and Franel numbers. These sums are double summations of hypergeometric terms. Using the telescoping method and certain mathematical software packages, we transform such a double summation into a single sum. With this new approach, we confirm several open conjectures of Sun.

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arXiv2017

Arithmetic properties of Delannoy numbers and Schröder numbers

Define $$D_n(x)=\sum_{k=0}^n\binom nk^2x^k(x+1)^{n-k}\ \ \ \mbox{for}\ n=0,1,2,\ldots$$ and $$s_n(x)=\sum_{k=1}^n\frac1n\binom nk\binom n{k-1}x^{k-1}(x+1)^{n-k}\ \ \ \mbox{for}\ n=1,2,3,\ldots.$$ Then $D_n(1)$ is the $n$-th central Delannoy number $D_n$, and $s_n(1)$ is the $n$-th little Schröder number $s_n$. In this paper we obtain some surprising arithmetic properties of $D_n(x)$ and $s_n(x)$. We show that $$\frac1n\sum_{k=0}^{n-1}D_k(x)s_{k+1}(x)\in\mathbb Z[x(x+1)]\ \quad\mbox{for all}\ n=1,2,3,\ldots.$$ Moreover, for any odd prime $p$ and $p$-adic integer $x\not\equiv0,-1\pmod p$, we establish the supercongruence $$\sum_{k=0}^{p-1}D_k(x)s_{k+1}(x)\equiv0\pmod{p^2}.$$ As an application we confirm Conjecture 5.5 in [S14a], in particular we prove that $$\frac1n\sum_{k=0}^{n-1}T_kM_k(-3)^{n-1-k}\in\mathbb Z\quad\mbox{for all}\ n=1,2,3,\ldots,$$ where $T_k$ is the $k$-th central trinomial coefficient and $M_k$ is the $k$-th Motzkin number.

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arXiv2017

Sums of four polygonal numbers with coefficients

Let $m\ge3$ be an integer. The polygonal numbers of order $m+2$ are given by $p_{m+2}(n)=m\binom n2+n$ $(n=0,1,2,\ldots)$. A famous claim of Fermat proved by Cauchy asserts that each nonnegative integer is the sum of $m+2$ polygonal numbers of order $m+2$. For $(a,b)=(1,1),(2,2),(1,3),(2,4)$, we study whether any sufficiently large integer can be expressed as $$p_{m+2}(x_1) + p_{m+2}(x_2) + ap_{m+2}(x_3) + bp_{m+2}(x_4)$$ with $x_1,x_2,x_3,x_4$ nonnegative integers. We show that the answer is positive if $(a,b)\in\{(1,3),(2,4)\}$, or $(a,b)=(1,1)\ \&\ 4\mid m$, or $(a,b)=(2,2)\ \&\ m\not\equiv 2\pmod 4$. In particular, we confirm a conjecture of Z.-W. Sun which states that any natural number can be written as $p_6(x_1) + p_6(x_2) + 2p_6(x_3) + 4p_6(x_4)$ with $x_1,x_2,x_3,x_4$ nonnegative integers.

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arXiv2017

Supercongruences involving dual sequences

In this paper we study some sophisticated supercongruences involving dual sequences. For $n=0,1,2,\ldots$ define $$d_n(x)=\sum_{k=0}^n\binom nk\binom xk2^k$$ and $$s_n(x)=\sum_{k=0}^n\binom nk\binom xk\binom{x+k}k=\sum_{k=0}^n\binom nk(-1)^k\binom xk\binom{-1-x}k.$$ For any odd prime $p$ and $p$-adic integer $x$, we determine $\sum_{k=0}^{p-1}(\pm1)^kd_k(x)^2$ and $\sum_{k=0}^{p-1}(2k+1)d_k(x)^2$ modulo $p^2$; for example, we establish the new $p$-adic congruence $$\sum_{k=0}^{p-1}(-1)^kd_k(x)^2\equiv(-1)^{\langle x\rangle_p}\pmod{p^2},$$ where $\langle x\rangle_p$ denotes the least nonnegative integer $r$ with $x\equiv r\pmod p$. For any prime $p>3$ and $p$-adic integer $x$, we determine $\sum_{k=0}^{p-1}s_k(x)^2$ modulo $p^2$ (or $p^3$ if $x\in\{0,\ldots,p-1\}$), and show that $$\sum_{k=0}^{p-1}(2k+1)s_k(x)^2\equiv0\pmod{p^2}.$$ We also pose several related conjectures.

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arXiv2017

Refining Lagrange's four-square theorem

Lagrange's four-square theorem asserts that any $n\in\mathbb N=\{0,1,2,\ldots\}$ can be written as the sum of four squares. This can be further refined in various ways. We show that any $n\in\mathbb N$ can be written as $x^2+y^2+z^2+w^2$ with $x,y,z,w\in\mathbb Z$ such that $x+y+z$ (or $x+2y$, $x+y+2z$) is a square (or a cube). We also prove that any $n\in\mathbb N$ can be written as $x^2+y^2+z^2+w^2$ with $x,y,z,w\in\mathbb N$ such that $P(x,y,z)$ is a square, whenever $P(x,y,z)$ is among the polynomials \begin{gather*} x,\ 2x,\ x-y,\ 2x-2y,\ a(x^2-y^2)\ (a=1,2,3),\ x^2-3y^2,\ 3x^2-2y^2, \\x^2+ky^2\ (k=2,3,5,6,8,12),\ (x+4y+4z)^2+(9x+3y+3z)^2, \\x^2y^2+y^2z^2+z^2x^2,\ x^4+8y^3z+8yz^3, x^4+16y^3z+64yz^3. \end{gather*} We also pose some conjectures for further research; for example, our 1-3-5-Conjecture states that any $n\in\mathbb N$ can be written as $x^2+y^2+z^2+w^2$ with $x,y,z,w\in\mathbb N$ such that $x+3y+5z$ is a square.

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arXiv2017

A new theorem on the prime-counting function

For $x>0$ let $π(x)$ denote the number of primes not exceeding $x$. For integers $a$ and $m>0$, we determine when there is an integer $n>1$ with $π(n)=(n+a)/m$. In particular, we show that for any integers $m>2$ and $a\le\lceil e^{m-1}/(m-1)\rceil$ there is an integer $n>1$ with $π(n)=(n+a)/m$. Consequently, for any integer $m>4$ there is a positive integer $n$ with $π(mn)=m+n$. We also pose several conjectures for further research; for example, we conjecture that for each $m=1,2,3,\ldots$ there is a positive integer $n$ such that $m+n$ divides $p_m+p_n$, where $p_k$ denotes the $k$-th prime.

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arXiv2016

On a pair of zeta functions

Let $m$ be a positive integer, and define $$ζ_m(s)=\sum_{n=1}^\infty\frac{(-e^{2πi/m})^{ω(n)}}{n^s}\ \ \ \ \text{and} \ \ \ \ ζ^*_m(s)=\sum_{n=1}^\infty\frac{(-e^{2πi/m})^{Ω(n)}}{n^s},$$ for $\Re(s)>1$, where $ω(n)$ denotes the number of distinct prime factors of $n$, and $Ω(n)$ represents the total number of prime factors of $n$ (counted with multiplicity). In this paper we study these two zeta functions and related arithmetical functions. We show that $$\sum^\infty_{n=1\atop n\ \text{is squarefree}}\frac{(-e^{2πi/m})^{ω(n)}}n=0\quad\text{if}\ \ m>4,$$ which is similar to the known identity $\sum_{n=1}^\inftyμ(n)/n=0$ equivalent to the Prime Number Theorem. For $m>4$, we prove that $$ζ_m(1):=\sum_{n=1}^\infty\frac{(-e^{2πi/m})^{ω(n)}}n=0 \ \ \ \ \text{and}\ \ \ \ ζ^*_m(1):=\sum_{n=1}^\infty\frac{(-e^{2πi/m})^{Ω(n)}}n=0.$$ We also raise a hypothesis on the parities of $Ω(n)-n$ which implies the Riemann Hypothesis.

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arXiv2016

On $x(ax+1)+y(by+1)+z(cz+1)$ and $x(ax+b)+y(ay+c)+z(az+d)$

In this paper we first investigate for what positive integers $a,b,c$ every nonnegative integer $n$ can be represented as $x(ax+1)+y(by+1)+z(cz+1)$ with $x,y,z$ integers. We show that $(a,b,c)$ can be either of the following seven triples: $$(1,2,3),\ (1,2,4),\ (1,2,5),\ (2,2,4),\ (2,2,5),\ (2,3,3),\ (2,3,4),$$ and conjecture that any triple $(a,b,c)$ among $$(2,2,6),\ (2,3,5),\ (2,3,7),\ (2,3,8),\ (2,3,9),\ (2,3,10)$$ also has the desired property. For integers $0\le b\le c\le d\le a$ with $a>2$, we prove that any nonnegative integer can be represented as $x(ax+b)+y(ay+c)+z(az+d)$ with $x,y,z$ integers, if and only if the quadruple $(a,b,c,d)$ is among $$(3,0,1,2),\ (3,1,1,2),\ (3,1,2,2),\ (3,1,2,3),\ (4,1,2,3).$$

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arXiv2016

Congruences involving $g_n(x)=\sum_{k=0}^n\binom nk^2\binom{2k}kx^k$

Define $g_n(x)=\sum_{k=0}^n\binom nk^2\binom{2k}kx^k$ for $n=0,1,2,...$. Those numbers $g_n=g_n(1)$ are closely related to Apéry numbers and Franel numbers. In this paper we establish some fundamental congruences involving $g_n(x)$. For example, for any prime $p>5$ we have $$\sum_{k=1}^{p-1}\frac{g_k(-1)}{k}\equiv 0\pmod{p^2}\quad{and}\quad\sum_{k=1}^{p-1}\frac{g_k(-1)}{k^2}\equiv 0\pmod p.$$ This is similar to Wolstenholme's classical congruences $$\sum_{k=1}^{p-1}\frac1k\equiv0\pmod{p^2}\quad{and}\quad\sum_{k=1}^{p-1}\frac{1}{k^2}\equiv0\pmod p$$ for any prime $p>3$.

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arXiv2016

On some universal sums of generalized polygonal numbers

For $m=3,4,\ldots$ those $p_m(x)=(m-2)x(x-1)/2+x$ with $x\in\mathbb Z$ are called generalized $m$-gonal numbers. Sun [13] studied for what values of positive integers $a,b,c$ the sum $ap_5+bp_5+cp_5$ is universal over $\mathbb Z$ (i.e., any $n\in\mathbb N=\{0,1,2,\ldots\}$ has the form $ap_5(x)+bp_5(y)+cp_5(z)$ with $x,y,z\in\mathbb Z$). We prove that $p_5+bp_5+3p_5\,(b=1,2,3,4,9)$ and $p_5+2p_5+6p_5$ are universal over $\mathbb Z$, as conjectured by Sun. Sun also conjectured that any $n\in\mathbb N$ can be written as $p_3(x)+p_5(y)+p_{11}(z)$ and $3p_3(x)+p_5(y)+p_7(z)$ with $x,y,z\in\mathbb N$; in contrast, we show that $p_3+p_5+p_{11}$ and $3p_3+p_5+p_7$ are universal over $\mathbb Z$. Our proofs are essentially elementary and hence suitable for general readers.

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arXiv2016

Problems on combinatorial properties of primes

For $x\ge0$ let $π(x)$ be the number of primes not exceeding $x$. The asymptotic behaviors of the prime-counting function $π(x)$ and the $n$-th prime $p_n$ have been studied intensively in analytic number theory. Surprisingly, we find that $π(x)$ and $p_n$ have many combinatorial properties which should not be ignored. In this paper we pose 60 open problems on combinatorial properties of primes (including connections between primes and partition functions) for further research. For example, we conjecture that for any integer $n>1$ one of the $n$ numbers $π(n),π(2n),...,π(n^2)$ is prime; we also conjecture that for any integer $n>6$ there exists a prime $p<n$ such that $pn$ is a primitive root modulo $p_n$. One of our conjectures involving the partition function $p(n)$ states that for any prime $p$ there is a primitive root $g<p$ modulo $p$ with $g\in\{p(n):\ n=1,2,3,...\}$.

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arXiv2016

Two congruences involving harmonic numbers with applications

The harmonic numbers $H_n=\sum_{0 3$ be a prime. With helps of some combinatorial identities, we establish the following two new congruences: $$\sum_{k=1}^{p-1}\frac{\binom{2k}k}kH_k\equiv\frac13\left(\frac p3\right)B_{p-2}\left(\frac13\right)\pmod{p}$$ and $$\sum_{k=1}^{p-1}\frac{\binom{2k}k}kH_{2k}\equiv\frac7{12}\left(\frac p3\right)B_{p-2}\left(\frac13\right)\pmod{p},$$ where $B_n(x)$ denotes the Bernoulli polynomial of degree $n$. As an application, we determine $\sum_{n=1}^{p-1}g_n$ and $\sum_{n=1}^{p-1}h_n$ modulo $p^3$, where $$g_n=\sum_{k=0}^n\binom nk^2\binom{2k}k\quad\mbox{and}\quad h_n=\sum_{k=0}^n\binom nk^2C_k$$ with $C_k=\binom{2k}k/(k+1)$.

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arXiv2016

Binomial coefficients, Catalan numbers and Lucas quotients

Let $p$ be an odd prime and let $a,m$ be integers with $a>0$ and $m \not\equiv0\pmod p$. In this paper we determine $\sum_{k=0}^{p^a-1}\binom{2k}{k+d}/m^k$ mod $p^2$ for $d=0,1$; for example, $$\sum_{k=0}^{p^a-1}\frac{\binom{2k}k}{m^k}\equiv\left(\frac{m^2-4m}{p^a}\right)+\left(\frac{m^2-4m}{p^{a-1}}\right)u_{p-(\frac{m^2-4m}{p})}\pmod{p^2},$$ where $(-)$ is the Jacobi symbol, and $\{u_n\}_{n\geqslant0}$ is the Lucas sequence given by $u_0=0$, $u_1=1$ and $u_{n+1}=(m-2)u_n-u_{n-1}$ for $n=1,2,3,\ldots$. As an application, we determine $\sum_{0<k<p^a,\, k\equiv r\pmod{p-1}}C_k$ modulo $p^2$ for any integer $r$, where $C_k$ denotes the Catalan number $\binom{2k}k/(k+1)$. We also pose some related conjectures.

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arXiv2016

New congruences involving products of two binomial coefficients

Let $p>3$ be a prime and let $a$ be a positive integer. We show that if $p\equiv1\pmod 4$ or $a>1$ then $$\sum_{k=0}^{\lfloor\frac34p^a\rfloor}\frac{\binom{2k}k^2}{16^k}\equivł(\frac{-1}{p^a}\r)\pmod{p^3}$$ with $(-)$ the Jacobi symbol, which confirms a conjecture of Z.-W. Sun. We also establish the following new congruences: \begin{align*}\sum_{k=0}^{(p-1)/2}\frac{\binom{2k}k\binom{3k}k}{27^k}\equiv&ł(\frac p3\r)\frac{2^p+1}3\pmod{p^2}, \\\sum_{k=0}^{(p-1)/2}\frac{\binom{6k}{3k}\binom{3k}k}{(2k+1)432^k}\equiv&ł(\frac p3\r)\frac{3^p+1}4\pmod{p^2}, \\\sum_{k=0}^{(p-1)/2}\frac{\binom{4k}{2k}\binom{2k}k}{(2k+1)64^k}\equiv&ł(\frac{-1}p\r)2^{p-1}\pmod{p^2}. \end{align*} Note that in 2003 Rodriguez-Villeguez posed conjectures on $$\sum_{k=0}^{p-1}\frac{\binom{2k}k^2}{16^k},\ \sum_{k=0}^{p-1}\frac{\binom{2k}k\binom{3k}k}{27^k},\ \sum_{k=1}^{p-1}\frac{\binom{4k}{2k}\binom{2k}k}{64^k},\ \sum_{k=1}^{p-1}\frac{\binom{6k}{3k}\binom{3k}k}{432^k}$$ modulo $p^2$ which were later proved.

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arXiv2016

New series for some special values of $L$-functions

Dirichlet's $L$-functions are natural extensions of the Riemann zeta function. In this paper we first give a brief survey of Apéry-like series for some special values of the zeta function and certain $L$-functions. Then, we establish two theorems on transformations of certain kinds of congruences. Motivated by the results and based on our computation, we pose 48 new conjectural series (most of which involve harmonic numbers) for such special values and related constants. For example, we conjecture that \begin{align*}\sum_{k=1}^\infty\frac1{k^4\binom{2k}k}\bigg(\frac1k+\sum_{j=k}^{2k}\frac1j\bigg)=&\frac{11}9ζ(5), \\\sum_{k=1}^\infty\frac{(-1)^{k-1}}{k^3\binom{2k}k}\bigg(\frac1{5k^3}+\sum_{j=1}^{k}\frac1{j^3}\bigg)=&\frac{2}5ζ(3)^2, \end{align*} and $$\sum_{k=1}^\infty\frac{48^k}{k(2k-1)\binom{4k}{2k}\binom{2k}k}=\frac{15}2\sum_{k=1}^\infty\frac{(\frac k3)}{k^2},$$ where $(\frac k3)$ denotes the Legendre symbol.

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arXiv2015

Natural numbers represented by $\lfloor x^2/a\rfloor+\lfloor y^2/b\rfloor+\lfloor z^2/c\rfloor$

Let $a,b,c$ be positive integers. It is known that there are infinitely many positive integers not representated by $ax^2+by^2+cz^2$ with $x,y,z\in\mathbb Z$. In contrast, we conjecture that any natural number is represented by $\lfloor x^2/a\rfloor+\lfloor y^2/b\rfloor +\lfloor z^2/c\rfloor$ with $x,y,z\in\mathbb Z$ if $(a,b,c)\not=(1,1,1),(2,2,2)$, and that any natural number is represented by $\lfloor T_x/a\rfloor+\lfloor T_y/b\rfloor+\lfloor T_z/c\rfloor$ with $x,y,z\in\mathbb Z$, where $T_x$ denotes the triangular number $x(x+1)/2$. We confirm this general conjecture in some special cases; in particular, we prove that $$\left\{x^2+y^2+\left\lfloor\frac{z^2}5\right\rfloor:\ x,y,z\in\mathbb Z\ \mbox{and}\ 2\nmid y\right\}=\{1,2,3,\ldots\}$$ and $$\left\{\left\lfloor\frac{x^2}m\right\rfloor+\left\lfloor\frac{y^2}m\right\rfloor+\left\lfloor\frac{z^2}m\right\rfloor:\ x,y,z\in\mathbb Z\right\} =\{0,1,2,\ldots\}\ \ \mbox{for}\ m=5,6,15.$$ We also pose several conjectures for further research; for example, we conjecture that any integer can be written as $x^4-y^3+z^2$, where $x$, $y$ and $z$ are positive integers.

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arXiv2015

A result similar to Lagrange's theorem

Generalized octagonal numbers are those $p_8(x)=x(3x-2)$ with $x\in\mathbb Z$. In this paper we mainly show that every positive integer can be written as the sum of four generalized octagonal numbers one of which is odd. This result is similar to Lagrange's theorem on sums of four squares. Moreover, for $35$ triples $(b,c,d)$ with $1\le b\le c\le d$ (including $(2,3,4)$ and $(2,4,8)$), we prove that any nonnegative integer can be exprssed as $p_8(w)+bp_8(x)+cp_8(y)+dp_8(z)$ with $w,x,y,z\in\mathbb Z$. We also pose several conjectures for further research.

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arXiv2015

The least modulus for which consecutive polynomial values are distinct

Let $d\ge4$ and $c\in(-d,d)$ be relatively prime integers. We show that for any sufficiently large integer $n$ (in particular $n>24310$ suffices for $4\le d\le 36$), the smallest prime $p\equiv c\pmod d$ with $p\ge(2dn-c)/(d-1)$ is the least positive integer $m$ with $2r(d)k(dk-c)\ (k=1,\ldots,n)$ pairwise distinct modulo $m$, where $r(d)$ is the radical of $d$. We also conjecture that for any integer $n>4$ the least positive integer $m$ such that $|\{k(k-1)/2\ \mbox{mod}\ m:\ k=1,\ldots,n\}|= |\{k(k-1)/2\ \mbox{mod}\ m+2:\ k=1,\ldots,n\}|=n$ is the least prime $p\ge 2n-1$ with $p+2$ also prime.

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