Divisibility properties of power GCD matrices and power LCM matrices

Let a,b and n be positive integers and the set S={x₁,…,x_n} of n distinct positive integers be a divisor chain (i.e. there exists a permutation σ on {1,…,n} such that x_σ(1)|…|x_σ(n)). In this paper, we show that if a|b, then the ath power GCD matrix (S^a) having the ath power (x_i,x_j)^a of the greatest common divisor of x_i and x_j as its i,j-entry divides the bth power GCD matrix (S^b) in the ring M_n(Z) of n×n matrices over integers. We show also that if a?b and n?2, then the ath power GCD matrix (S^a) does not divide the bth power GCD matrix (S^b) in the ring M_n(Z). Similar results are also established for the power LCM matrices.     

Representations of integers by linear forms in nonnegative integers

Let Ω be the set of positive integers that are omitted values of the form f = Σ_i=1ⁿa_ix_i, where the a_i are fixed and relatively prime natural numbers and the x_i are variable nonnegative integers. Set ω = #Ω and κ = max Ω + 1 (the conductor). Properties of ω and κ are studied, such as an estimate for ω (similar to one found by Brauer) and the inequality 2ω ≥ κ. The so-called Gorenstein condition is shown to be equivalent to 2ω = κ.     

Testing integer knapsacks for feasibility

We present a new approach for determining whether there exist nonnegative integers x₁, x₂, …, x_n satisfying a₁x₁ + a₂x₂ + ? + a_nx_n = b, where a₁ < a₂ < ? < a_n and b are nonnegative integers. case time complexity is analyzed and compared with dynamic programming techniques. Computational results are given.     

On discount residue classes

It is shown that, whenever m₁, m₂,…, m_n are natural numbers such that the pairwise greatest common divisors, d_ij=(m_i, m_j), i≠j are distinct and different from 1, then there exist integers a₁, a₂,…,a_n such that the solution sets of the congruences x≡_i (modm_i), i= 1,2,…,n are disjoint.     

The Erd?s-Ginzberg-Ziv theorem with units

Let x₁,…,x_r be a sequence of elements of Z_n, the integers modulo n. How large must r be to guarantee the existence of a subsequence x_i1,…,x_in and units α₁,…,α_n with α₁x_i1+?+α_nx_in=0? Our main aim in this paper is to show that r=n+a is large enough, where a is the sum of the exponents of primes in the prime factorisation of n. This result, which is best possible, could be viewed as a unit version of the Erd?s-Ginzberg-Ziv theorem. This proves a conjecture of Adhikari, Chen, Friedlander, Konyagin and Pappalardi.We also discuss a number of related questions, and make conjectures which would greatly extend a theorem of Gao.     

A binary linear recurrence sequence of composite numbers

Let (a,b)∈Z², where b≠0 and (a,b)≠(±2,−1). We prove that then there exist two positive relatively prime composite integers x₁, x₂ such that the sequence given by xn+1=axn+bxn−1, n=2,3,… , consists of composite terms only, i.e., |xn| is a composite integer for each n∈N. In the proof of this result we use certain covering systems, divisibility sequences and, for some special pairs (a,±1), computer calculations. The paper is motivated by a result of Graham who proved this theorem in the special case of the Fibonacci-like sequence, where (a,b)=(1,1).     

Siegel’s Lemma and Sum-Distinct Sets

Let L(x)=a ₁ x ₁+a ₂ x ₂+???+a _n x _n , n≥2, be a linear form with integer coefficients a ₁,a ₂,…,a _n which are not all zero. A basic problem is to determine nonzero integer vectors x such that L(x)=0, and the maximum norm ‖x‖ is relatively small compared with the size of the coefficients a ₁,a ₂,…,a _n . The main result of this paper asserts that there exist linearly independent vectors x ₁,…,x _n?1∈? ⁿ such that L(x _i )=0, i=1,…,n?1, and $$\|{\mathbf{x}}_{1}\|\cdots\|{\mathbf{x}}_{n-1}\|<\frac{\|{\mathbf{a}}\|}{\sigma_{n}},$$ where a=(a ₁,a ₂,…,a _n ) and $$\sigma_{n}=\frac{2}{\pi}\int_{0}^{\infty}\left(\frac{\sin t}{t}\right)^{n}\,dt.$$ This result also implies a new lower bound on the greatest element of a sum-distinct set of positive integers (Erdös–Moser problem). The main tools are the Minkowski theorem on successive minima and the Busemann theorem from convex geometry.     

Divisibility of determinants of power GCD matrices and power LCM matrices on finitely many quasi-coprime divisor chains

Let a, n ? 1 be integers and S = {x₁, … , x_n} be a set of n distinct positive integers. The matrix having the ath power (x_i, x_j)^a of the greatest common divisor of x_i and x_j as its i, j-entry is called ath power greatest common divisor (GCD) matrix defined on S, denoted by (S^a). Similarly we can define the ath power LCM matrix [S^a]. We say that the set S consists of finitely many quasi-coprime divisor chains if we can partition S as S = S₁ ∪ ? ∪ S_k, where k ? 1 is an integer and all S_i (1 ? i ? k) are divisor chains such that (max(S_i), max(S_j)) = gcd(S) for 1 ? i ≠ j ? k. In this paper, we first obtain formulae of determinants of power GCD matrices (S^a) and power LCM matrices [S^a] on the set S consisting of finitely many quasi-coprime divisor chains with gcd(S) ∈ S. Using these results, we then show that det(S^a)∣det(S^b), det[S^a]∣det[S^b] and det(S^a)∣det[S^b] if a∣b and S consists of finitely many quasi-coprime divisor chains with gcd(S) ∈ S. But such factorizations fail to be true if such divisor chains are not quasi-coprime.     

Note on the local growth of iterated polynomials

The local behavior of the iterates of a real polynomial is investigated. The fundamental result may be stated as follows: THEOREM. Let x_i, for i=1, 2, ..., n+2, be defined recursively by x_i+1=f(x_i), where x₁ is an arbitrary real number and f is a polynomial of degree n. Let x_i+1?x_i≧1 for i=1, ..., n + 1. Then for all i, 1 ≦i≦n, and all k, 1≦k≦n+1?i, $$ - \frac{{2^{k - 1} }}{{k!}}< f\left[ {x_1 ,... + x_{i + k} } \right]< \frac{{x_{i + k + 1} - x_{i + k} + 2^{k - 1} }}{{k!}},$$ where f[x_i, ..., x_i+k] denotes the Newton difference quotient. As a consequence of this theorem, the authors obtain information on the local behavior of the solutions of certain nonlinear difference equations. There are several cases, of which the following is typical: THEOREM. Let {x_i}, i = 1, 2, 3, ..., be the solution of the nonlinear first order difference equation x_i+1=f(x_i) where x₁ is an arbitrarily assigned real number and f is the polynomial \(f(x) = \sum\limits_{j = 0}^n {a_j x^j } ,n \geqq 2\) . Let δ be positive with δ^n?1=|2^n?1/n!a_n|. Then, if n is even and a_n<0, there do not exist n + 1 consecutive increments Δx_i=x_i+1?x_i in the solution {x_i} with Δx_i≧δ. The special case in which the iterated polynomial has integer coefficients leads to a “nice” upper bound on a generalization of the van der Waerden numbers. Ap _k -sequence of length n is defined to be a strictly increasing sequence of positive integers {x ₁, ...,x _n } for which there exists a polynomial of degree at mostk with integer coefficients and satisfyingf(x _j )=x _j+1 forj=1, 2, ...,n?1. Definep _k (n) to be the least positive integer such that if {1, 2, ...,p _k (n)} is partitioned into two sets, then one of the two sets must contain ap _k -sequence of lengthn. THEOREM. p_n?2(n)≦(n!)^(n?2)!/2.     

Segmental solutions of Boolean equations

Davio and Deschamps have shown that the solution set, K, of a consistent Boolean equation $\text{?(x}{}_1\text{, …, x}{}_{\mathrm{n}}\text{)=0}$ over a finite Boolean algebra B may be expressed as the union of a collection of subsets of Bⁿ, each of the form {(x₁, …, x_n) | a_i≤x_i≤b_i, a_i?B, b_i?B, i = 1, …, n}. We refer to such subsets of Bⁿ as segments and to the collection as a segmental cover of K. We show that $\text{?(x}{}_1\text{, …, x}{}_{\mathrm{n}}\text{) = 1}$ is consistent if and only if ? can be expressed by one of a class of sum-of-products expressions which we call segmental formulas. The object of this paper is to relate segmental covers of K to segmental formulas for ?.     

On the representation of integers by p-adic diagonal forms

Let p be an odd prime, let d be a positive integer such that (d,p?1)=1, let r denote the p-adic valuation of d and let m=1+3+3²+…+3^r. It is shown that for every p-adic integer n the equation Σ_i=1^mX_i^d=n has a nontrivial p-adic solution. It is also shown that for all p-adic units a₁, a₂, a₃, a₄ and all p-adic integers n the equation Σ_i=1⁴a_iX_i^p=n has a nontrivial p-adic solution. A corollary to each of these results is that every p-adic integer is a sum of four pth powers of p-adic integers.     

Combinational properties of sets of residues modulo a prime and the Erdős—Graham problem

Consider an arbitrary ε > 0 and a sufficiently large prime p > 2. It is proved that, for any integer a, there exist pairwise distinct integers x ₁, x ₂, ..., x _N , where N = 8([1/ε + 1/2] + 1)² such that 1 ≤ x _i ≤ p ^ε, i = 1, ..., N, and $$a \equiv x_1^{ - 1} + \cdots + x_N^{ - 1} (\bmod p)$$ , where x _i ^?1 is the least positive integer satisfying x _i ^?1 x _i ≡ 1 (modp). This improves a result of Sparlinski.     

On diophantine equations of the form {({\boldsymbol x}-{\boldsymbol a}_{\bf 1})({\boldsymbol x}-{\boldsymbol a}_{\bf 2}) \ldots ({\boldsymbol x}-{\boldsymbol a}_{\boldsymbol k}) + {\boldsymbol r} = {\boldsymbol y}^{ \boldsymbol n}}

Erd?s and Selfridge [3] proved that a product of consecutive integers can never be a perfect power. That is, the equation x(x?+?1)(x?+?2)...(x?+?(m???1))?=?y ⁿ has no solutions in positive integers x,m,n where m, n?>?1 and y?∈?Q. We consider the equation $$ (x-a_1)(x-a_2) \ldots (x-a_k) + r = y^n $$ where 0?≤?a ₁?<?a ₂?<???<?a _k are integers and, with r?∈?Q, n?≥?3 and we prove a finiteness theorem for the number of solutions x in Z, y in Q. Following that, we show that, more interestingly, for every nonzero integer n?>?2 and for any nonzero integer r which is not a perfect n-th power for which the equation admits solutions, k is bounded by an effective bound.     

On the g-circulant solutions to the matrix equation Am = λJ

Let g and n be positive integers and let a₁ = (g, n) and a_m = (g^m, n). If h ≡ 0 (mod $\text{na}{}_1\text{a}{}_{\mathrm{m}}$), then the g-circulant whose Hall polynomial is equal to Σ_i=0^h?1xⁱ satisfies the matrix equation A^m = λJ, where n is the size of matrix J.     

Arithmetic behaviour of the sums of three squares

Let n ≠ 4^a(8b + 7) be an integer. We deal with the problem of the solvability of the equation n = x₁² + x₂² + x₃² in integers x₁, x₂, x₃ prime to n. By a theorem of Vila (Arch. Math. 44 (1985), 424–437), the existence of such a solution implies that every central extension of the alternating group A_n, for n ≡ 3 (mod 8), can be realized as a Galois group over Q.     

Polynomials arising in factoring generalized Vandermonde determinants II: A condition for monicity

In our previous paper [1], we observed that generalized Vandermonde determinants of the form V_n;ν(x₁,…,x_s) = |x_i^μk|, 1 ≤ i, k ≤ n, where the x_i are distinct points belonging to an interval [a, b] of the real line, the index n stands for the order, the sequence μ consists of ordered integers 0 ≤ μ₁ < μ₂ < … < μ_n, can be factored as a product of the classical Vandermonde determinant and a Schur function. On the other hand, we showed that when x = x_n, the resulting polynomial in x is a Schur function which can be factored as a two-factors polynomial: the first is the constant ∏_i=1ⁿ⁻¹ x_i^μ₁ times the monic polynomial ∏_i=1ⁿ⁻¹ (x − x_i, while the second is a polynomial P_M(x) of degree M = m_n−1 − n + 1. In this paper, we first present a typical application in which these factorizations arise and then we discuss a condition under which the polynomial P_M (x) is monic.     

The periods of the sequences generated by some symmetric shift registers

E_k(x₂,…, x_n) is defined by E_k(a₂,…, a_n) = 1 if and only if ∑_i=2ⁿa_i = k. We determine the periods of sequences generated by the shift registers with the feedback functions x₁ + E_k(x₂,…, x_n) and x₁ + E_k(x₂,…, x_n) + E_k+1(x₂,…, x_n) over the field GF(2).     

On a conjecture of Chowla and Chowla

The equation y² ≡ x(x + a₁)(x + a₂) … (x + a_r) (mod p), where a₁, a₂, …, a_r are integers is shown to have a solution in integers x, y with 1 ≦ x ≦C, where C is a constant depending only on a₁, a₂, …, a_r.     

Permanence and Positive Periodic Solutions for Kolmogorov Competing Species Systems

We prove the existence of periodic solutions in a compact attractor of (R⁺)ⁿ for the Kolmogorov system x′_i = x_if_i(t, x₁, …, x_n), i = l, …, n in the competitive case. Extension to differential delay equations are con- sidered too. Applications are given to Lotka-Volterra systems with periodic coefficients.     

类对称函数的Schur凸性和不等式

对x = (x₁, x₂,···, x_n) ∈ (0,1)ⁿ 和 r ∈ {1, 2,···, n} 定义对称函数 F_n(x, r) = F_n(x₁, x₂,···, x_n; r) =∏_{1≤i1∑j=1^r(1+xi3/1- xi3)^1/r, 其中i1, i2, ···, ir 是整数. 该文证明了Fn(x, r) 是(0,1)ⁿ 上的Schur凸、Schur乘性凸和Schur调和凸函数. 作为应用,利用控制理论建立了若干不等式.}