Infinite families of 2- and 3-designs with parameters v = p + 1, k = (p − 1)/2i + 1, where p odd prime, 2e T (p − 1), e ≥ 2, 1 ≤ i ≤ e

Let p be an odd prime number such that p − 1 = 2^em for some odd m and e ≥ 2. In this article, by using the special linear fractional group PSL(2, p), for each i, 1 ≤ i ≤ e, except particular cases, we construct a 2-design with parameters v = p + 1, k = (p − 1)/2ⁱ + 1 and λ = ((p − 1)/2ⁱ+1)(p − 1)/2 = k(p − 1)/2, and in the case i = e we show that some of these 2-designs are 3-designs. Likewise, by using the linear fractional group PGL(2,p) we construct an infinite family of 3-designs with the same v k and λ = k(k − 2). These supplement a part of [4], in which we gave an infinite family of 3-designs with parameters v = q + 1, k = (q + 1)/2 = (q − 1)/2 + 1 and λ = (q + 1)(q − 3)/8 = k(k − 2)/2, where q is a prime power such that q − 1 = 2m for some odd m and q > 7. Some of the designs given in this article and in [4] fill in a few blanks in the table of Chee, Colbourn, and Kreher [2]. © 1997 John Wiley & Sons, Inc.     

Quasi‐symmetric designs with fixed difference of block intersection numbers

The following results for proper quasi‐symmetric designs with non‐zero intersection numbers x,y and λ > 1 are proved.

• (1) Let D be a quasi‐symmetric design with z = y ? x and v ≥ 2k. If x ≥ 1 + z + z³ then λ < x + 1 + z + z³.
• (2) Let D be a quasi‐symmetric design with intersection numbers x, y and y ? x = 1. Then D is a design with parameters v = (1 + m) (2 + m)/2, b = (2 + m) (3 + m)/2, r = m + 3, k = m + 1, λ = 2, x = 1, y = 2 and m = 2,3,… or complement of one of these design or D is a design with parameters v = 5, b = 10, r = 6, k = 3, λ = 3, and x = 1, y = 2.
• (3) Let D be a triangle free quasi‐symmetric design with z = y ? x and v ≥ 2k, then x ≤ z + z².
• (4) For fixed z ≥ 1 there exist finitely many triangle free quasi‐symmetric designs non‐zero intersection numbers x, y = x + z.
• (5) There do not exist triangle free quasi‐symmetric designs with non‐zero intersection numbers x, y = x + 2.

© 2006 Wiley Periodicals, Inc. J Combin Designs 15: 49–60, 2007     

Representation of Isotropic Turbulence by Scalar Functions

The maximum number N(n) of scalar functions of wavevector needed to represent the general cumulant of order n of isotropic turbulence is determined. In addition to reproducing the known results that N(2) = 1, N(3) = 2, it is shown that N(4) = 4, N(5) = 4, N(6) = 9, etc.     

Covering the vertices of a graph by cycles of prescribed length

The main theorem of that paper is the following: let G be a graph of order n, of size at least (n² - 3n + 6)/2. For any integers k, n₁, n₂,…,n_k such that n = n₁ + n₂ +. + n_k and n_i ? 3, there exists a covering of the vertices of G by disjoint cycles (C_i) =l…k with |C_i| = ni, except when n = 6, n₁ = 3, n₂ = 3, and G is isomorphic to G₁, the complement of G₁ consisting of a C₃ and a stable set of three vertices, or when n = 9, n₁ = n₂ = n₃ = 3, and G is isomorphic to G₂, the complement of G₂ consisting of a complete graph on four vertices and a stable set of five vertices. We prove an analogous theorem for bipartite graphs: let G be a bipartite balanced graph of order 2n, of size at least n² - n + 2. For any integers s, n₁, n₂,…,n_s with n_i ? 2 and n = n₁ + n₂ + ? + n_s, there exists a covering of the vertices of G by s disjoint cycles C_i, with |C_i| = 2n_i.     

Edge disjoint cycles in graphs

Ore proved in 1960 that if G is a graph of order n and the sum of the degrees of any pair of nonadjacent vertices is at least n, then G has a hamiltonian cycle. In 1986, Li Hao and Zhu Yongjin showed that if n ? 20 and the minimum degree δ is at least 5, then the graph G above contains at least two edge disjoint hamiltonian cycles. The result of this paper is that if n ? 2δ², then for any 3 ? l₁ ? l₂ ? ? ? l_k ? n, 1 = k = [(δ - 1)/2], such graph has K edge disjoint cycles with lengths l₁, l₂…l_k, respectively. In particular, when l₁ = l₂ = ? = l_k = n and k = [(δ - 1)/2], the graph contains [(δ - 1)/2] edge disjoint hamiltonian cycles.     

Über multiplikative Funktionen und die daraus entspringenden Differentialsysteme

Zusammenfassung Den Angelpunkt der folgenden Untersuchungen bilden die im Abschnitt II bewiesenenPeriodenrelationen für relativ zur schlichten Ebene nur multiplikativ verzweigte Funktionen der Gestalt: ( _j und _j nicht ganzzahlig, sonst beliebig komplex;R(z) rational mit Polen höchstens beia _j und ). Faßt man bei festena _j , _j alle diese Funktionen zu einer Klassek ₀ zusammen, die durch Ersetzung von _j durch – _j entstehenden zu der komplementären Klassek ₀, so gibt es 1. ink ₀ bzw.k ₀ jen Basisfunktionen derart, daß sich jede Funktion der Klasse linear homogen mit konstanten Koeffizienten aus diesen und der Ableitung einer Klassenfunktion darstellen läßt, 2. jen Basiswege, so beschaffen, daß sich das Integral irgendeiner Klassenfunktion über einen geschlossenen Weg linear homogen mit Koeffizienten aus einem gewissen Ring aus den betreffenden speziellen Integralen aufbaut. Durch Integration der Basisfunktionen über die Basiswege entstehen zweiquadratische Periodenschemata, zwischen deren Elementenn ² bilineare Relationen bestehen; diese können als Verallgemeinerung der Periodenrelationen im hyperelliptischen algebraischen Fall angesehen werden.Insbesondere werden die Integrale als Funktionen eines Parametersu betrachtet, von dem die Verzweigungspunkte in gewisser Weise algebraisch abhängen sollen. Jedes der obigen Periodenschemata gibt dabei ein Fundamentalsystem je eines Differentialsystems mit rationalen Koeffizienten. Die Periodenrelationen liefern für diese folgenden Hauptsatz: das eine der beiden Differentialsysteme istmit dem adjungierten des andern von der gleichen Art, oder etwas schärfer: bei geeigneter Wahl der Basiswege liefern die beiden Fundamentalsysteme (abgesehen von leicht angebbaren Faktoren) Basissysteme zueinanderkomplementärer Riemannscher Klassen; das besagt: die zu den einzelnen singulären Punkten gehörigen UmlaufssubstitutionenM, M sind zueinander komplementär:M =M (M=Transponiete vonM ^–1). Dadurch scheint die klassische, bei Riemann (Ges. Werke, 2. A., S. 67–83) für den Gaußschen hypergeometrischen Fall angedeutete und inzwischen in Sonderfällen mehrfach behandelte Frage nach den durch dieIntegrale mit entgegengesetzten Exponenten dargestellten Funktionen in angemessener Allgemeinheit erledigt zu sein.     

Repelling periodic points of given periods of rational functions

Let R(z) be a rational function of degree d≥2. Then R(z) has at least one repelling periodic point of given period k≥2, unless k = 4 and d = 2, or k = 3 and d≤3, or k = 2 and d≤8. Examples show that all exceptional cases occur.     

Bemerkung über das Flächennetz zweiter Ordnung

Ohne ZusammenfassungWenn man symbolisch schreibt =a _x ² , =b _x ² , =c _x ² , so ergiebt sich aus Herrn Gordan's wichtigem Combinantensatze, dass alle Combinanten des Netzes simultane Invarianten der zwei Formen und (abuv) (acuv) (bcuv) (abcu)a _x2 b _x2 c _x2 sind; man findet dies in evidenter Weise durch Einsicht in Herrn Sturm's schöne Arbeit bestätigt (Borchardt's Journal Bd. 70). Die ganze algebraische Theorie aber ist mit nicht geringen Schwierigkeiten verknüpft, obwohl die nicht veröffentlichten Resultate des Herrn Gundelfinger erlauben, eine Reihe von Sätzen aus den ebenen Kegelschnittnetzen zu übertragen.     

Approximation of the Jacobian matrix in (<Emphasis Type="Italic">m</Emphasis>, 2)-methods for solving stiff problems

An initial value problem for stiff systems of first-order ordinary differential equations is considered. In the class of (m, k)-methods, two integration algorithms with a variable step size based on second (m = k = 2) and third (k = 2, m = 3) order-accurate schemes are constructed in which both analytical and numerical Jacobian matrices can be frozen. A theorem on the maximum order of accuracy of (m, 2)-methods with a certain approximation of the Jacobian matrix is proved. Numerical results are presented.     

Analogues to Fermat primes related to Pell��s equation

The coefficients of some weight 3 modular forms give reason to study primes of the form p = 2x ² ? 1 = 2dy ² + 1. If x _a , y _a are the positive solutions of Pell??s equation x ² ? dy ² = 1, given by ${x_a + y_a \sqrt{d} = (x_1 + y_1 \sqrt{d})^a}The coefficients of some weight 3 modular forms give reason to study primes of the form p = 2x ² − 1 = 2dy ² + 1. If x _a , y _a are the positive solutions of Pell’s equation x ² − dy ² = 1, given by x_a + y_a ?d = (x₁ + y₁ ?d)^a{x_a + y_a \sqrt{d} = (x_1 + y_1 \sqrt{d})^a}, and if p_a = 2 x_a² - 1{p_a = 2 x_a^2 - 1} is prime, then a = 2 ^m is a power of 2. So there are analogues to the Fermat numbers 2 ^a + 1.     

Applications of balance equations to vertex switching reconstruction

A graph is called s-vertex switching reconstructible (s-VSR) if it is uniquely defined, up to isomorphism, by the multiset of unlabeled graphs obtained by switching of all its s-vertex subsets. We show that a graph with n vertices is n/2-VSR if n = 0(mod 4), (n ? 2)/2-VSR and n/2-VSR if n = 2(mod 4), (n ? 1)/2-VSR if n = 1 (mod 2). For hypothetical nonreconstructible graphs, we give bounds on the number of edges (for any s) and on the maximum and minimum degree (for s = 2). We also show that for n > 9 the degree sequence is 2-VSR.     

On the Upper Bounds of the Numbers of Perfect Matchings in Graphs with Given Parameters

Let φ(G),κ(G),α(G),χ(G),cl(G),diam(G)denote the number of perfect matchings,connectivity,independence number,chromatic number,clique number and diameter of a graph G,respectively.In this note,by constructing some extremal graphs,the following extremal problems are solved:1.max{φ(G):|V(G)|=2n,κ(G)≤k}=k[(2n-3)!!],2.max{φ(G):|V(G)|=2n,α(G)≥k}=[multiply from i=0 to k-1(2n-k-i)[(2n-2k-1)!!],3.max{φ(G):|V(G)|=2n,χ(G)≤k}=φ(T_(k,2n))T_(k,2n)is the Turán graph,that is a complete k-partite graphon 2n vertices in which all parts are as equal in size as possible,4.max{φ(G):|V(G)|=2n,cl(G)=2}=n1,5.max{φ(G):|V(G)|=2n,diam(G)≥2}=(2n-2)(2n-3)[(2n-5)!!],max{φ(G):|V(G)|=2n,diam(G)≥3}=(n-1)~2[(2n-5)!!].     

The moduli component of the space of semistable rank-2 sheaves on ℙ3 with singularities of mixed dimension

A new irreducible component of the Gieseker–Maruyama moduli scheme M(3) of semistable coherent sheaves of rank 2 with Chern classes c₁ = 0, c₂ = 3, and c₃ = 0 on P³ such that its general point corresponds to a sheaf whose singular locus contains components of dimensions 0 and 1 is described. These sheaves are obtained by elementary transformations of stable reflexive sheaves of rank 2 with Chern classes c₁ = 0, c₂ = 2, and c₃ = 2 along the projective line. The constructed family of sheaves is the first example of an irreducible component of a Gieseker–Maruyama scheme whose general point corresponds to a sheaf with singularities of mixed dimension.

     

Boundedness of solutions for a class of oscillating potentials without the twist condition

In this paper, we prove that the second order differential equation d^2x/dt^2＋x^2n_1f（x）＋p（t）=0with p（t ＋ 1） = p（t）, f（x ＋ T） = f（x） smooth and f（x） 〉 0, possesses Lagrangian stability despite of the fact that the monotone twist condition is violated.     

On the ratio of two blocks of consecutive integers

Under certain assumptions, it is shown that eq. (2) has only finitely many solutions in integersx≥0,y≥0,k≥2,l≥0. In particular, it is proved that (2) witha=b=1, l=k implies thatx=7,y=0,k=3.     

Globally generated vector bundles on with

One classifies the globally generated vector bundles on with the first Chern class c₁ = 3. The case c₁ = 1 is very easy, the case c₁ = 2 was done in [42], the case c₁ = 3, rank =2 was settled in [21] and the case c₁ ≤ 5, rank = 2 in [10]. Our work is based on Serre's theorem relating vector bundles of rank = 2 with codimension 2 lci subschemes and its generalization for higher ranks, considered firstly by Vogelaar in [48].     

On subgroups of finite <Emphasis Type="Italic">p</Emphasis>-groups

In §2, we prove that if a 2-group G and all its nonabelian maximal sub-groups are two-generator, then G is either metacyclic or minimal non-abelian. In §3, we consider a similar question for p > 2. In §4 the 2-groups all of whose minimal nonabelian subgroups have order 16 and a cyclic subgroup of index 2, are classified. It is proved, in §5, that if G is a nonmetacyclic two-generator 2-group and A, B, C are all its maximal subgroups with d(A) ≤ d(B) ≤ d(C), then d(C) = 3 and either d(A) = d(B) = 3 (this occurs if and only if G/G′ has no cyclic subgroup of index 2) or else d(A) = d(B) = 2. Some information on the last case is obtained in Theorem 5.3.     

A Class of Gorenstein Artin Algebras of Embedding Dimension Four

In this article, we study height four graded Gorenstein ideals I in k[x, y, z, w] such that I ₂ is of height one and generated by three quadrics. After a suitable linear change of variables, I ∩ k[x, y, z] is either Gorenstein or of type two. The former case was studied by Iarrobino and Srinivasan [8 Iarrobino , A. , Srinivasan , H. ( 2005 ). Artininan Gorenstein algebras of embedding dimension four: components of ? Gor (H) for H = (1, 4, 7,…, 1) . Journal of Pure and Applied Algebra 201 : 62 – 96 .[Crossref], [Web of Science ®] , [Google Scholar]] where they give the structure of the ideal and its resolution. We study the latter case and give the structure of these ideals and their minimal resolution. We also explicitly write the form of the generators of I and the maps in the free resolution of R/I.     

Disjoint skolem sequences and related disjoint structures

A Skolem sequence of order n is a sequence S = (s₁, s₂…, s²ⁿ) of 2n integers satisfying the following conditions: (1) for every k ∈ {1, 2,… n} there exist exactly two elements s_i,S_j such that S_i = S_j = k; (2) If s_i = s_j = k,i < j then j ? i = k. In this article we show the existence of disjoint Skolem, disjoint hooked Skolem, and disjoint near-Skolem sequences. Then we apply these concepts to the existence problems of disjoint cyclic Steiner and Mendelsohn triple systems and the existence of disjoint 1-covering designs. © 1993 John Wiley & Sons, Inc.