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Octonion multiplication and Heawood’s map

Bruno Sévennec (2013)

Confluentes Mathematici

In this note, the octonion multiplication table is recovered from a regular tesselation of the equilateral two timensional torus by seven hexagons, also known as Heawood’s map.

Od unimodálních posloupností k narozeninovému paradoxu

Antonín Slavík (2016)

Pokroky matematiky, fyziky a astronomie

Konečná posloupnost reálných čísel se nazývá unimodální, pokud ji lze rozdělit na neklesající a nerostoucí úsek. V textu se zaměříme především na kombinatorické posloupnosti tvořené kombinačními čísly nebo Stirlingovými čísly prvního a druhého druhu. Kromě unimodality se budeme věnovat též příbuznému pojmu logaritmické konkávnosti. Ukážeme, jak tato témata souvisejí s klasickými Newtonovými a Maclaurinovými nerovnostmi, které v závěru využijeme k řešení obecné verze narozeninového paradoxu.

OD-characterization of almost simple groups related to L 2 ( 49 )

Liang Cai Zhang, Wu Jie Shi (2008)

Archivum Mathematicum

In the present paper, we classify groups with the same order and degree pattern as an almost simple group related to the projective special linear simple group L 2 ( 49 ) . As a consequence of this result we can give a positive answer to a conjecture of W. J. Shi and J. X. Bi, for all almost simple groups related to L 2 ( 49 ) except L 2 ( 49 ) · 2 2 . Also, we prove that if M is an almost simple group related to L 2 ( 49 ) except L 2 ( 49 ) · 2 2 and G is a finite group such that | G | = | M | and Γ ( G ) = Γ ( M ) , then G ≅ M .

Odd and residue domination numbers of a graph

Yair Caro, William F. Klostermeyer, John L. Goldwasser (2001)

Discussiones Mathematicae Graph Theory

Let G = (V,E) be a simple, undirected graph. A set of vertices D is called an odd dominating set if |N[v] ∩ D| ≡ 1 (mod 2) for every vertex v ∈ V(G). The minimum cardinality of an odd dominating set is called the odd domination number of G, denoted by γ₁(G). In this paper, several algorithmic and structural results are presented on this parameter for grids, complements of powers of cycles, and other graph classes as well as for more general forms of "residue" domination.

Odd cutsets and the hard-core model on ℤ d

Ron Peled, Wojciech Samotij (2014)

Annales de l'I.H.P. Probabilités et statistiques

We consider the hard-core lattice gas model on ℤ d and investigate its phase structure in high dimensions. We prove that when the intensity parameter exceeds C d - 1 / 3 ( log d ) 2 , the model exhibits multiple hard-core measures, thus improving the previous bound of C d - 1 / 4 ( log d ) 3 / 4 given by Galvin and Kahn. At the heart of our approach lies the study of a certain class of edge cutsets in ℤ d , the so-called odd cutsets, that appear naturally as the boundary between different phases in the hard-core model. We provide a refined combinatorial...

Offensive alliances in graphs

Odile Favaron, Gerd Fricke, Wayne Goddard, Sandra M. Hedetniemi, Stephen T. Hedetniemi, Petter Kristiansen, Renu C. Laskar, R. Duane Skaggs (2004)

Discussiones Mathematicae Graph Theory

A set S is an offensive alliance if for every vertex v in its boundary N(S)- S it holds that the majority of vertices in v's closed neighbourhood are in S. The offensive alliance number is the minimum cardinality of an offensive alliance. In this paper we explore the bounds on the offensive alliance and the strong offensive alliance numbers (where a strict majority is required). In particular, we show that the offensive alliance number is at most 2/3 the order and the strong offensive alliance number...

On 1-dependent ramsey numbers for graphs

E.J. Cockayne, C.M. Mynhardt (1999)

Discussiones Mathematicae Graph Theory

A set X of vertices of a graph G is said to be 1-dependent if the subgraph of G induced by X has maximum degree one. The 1-dependent Ramsey number t₁(l,m) is the smallest integer n such that for any 2-edge colouring (R,B) of Kₙ, the spanning subgraph B of Kₙ has a 1-dependent set of size l or the subgraph R has a 1-dependent set of size m. The 2-edge colouring (R,B) is a t₁(l,m) Ramsey colouring of Kₙ if B (R, respectively) does not contain a 1-dependent set of size l (m, respectively); in this...

On 2 -extendability of generalized Petersen graphs

Nirmala B. Limaye, Mulupuri Shanthi C. Rao (1996)

Mathematica Bohemica

Let G P ( n , k ) be a generalized Petersen graph with ( n , k ) = 1 , n > k ≥ 4 . Then every pair of parallel edges of G P ( n , k ) is contained in a 1-factor of G P ( n , k ) . This partially answers a question posed by Larry Cammack and Gerald Schrag [Problem 101, Discrete Math. 73(3), 1989, 311-312].

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