Half domination arrangements in regular and semi-regular tessellation type graphs

We study the problem of half-domination sets of vertices in vertex transitive infinite graphs generated by regular or semi-regular tessellations of the plane. In some cases, the results obtained are sharp and in the rest, we show upper bounds for the average densities of vertices in half-domination sets.Comment: 14 pages, 12 figure

Minimum rank and zero forcing number for butterfly networks

The minimum rank of a simple graph $G$ is the smallest possible rank over all symmetric real matrices $A$ whose nonzero off-diagonal entries correspond to the edges of $G$. Using the zero forcing number, we prove that the minimum rank of the butterfly network is $\frac19\left[(3r+1)2^{r+1}-2(-1)^r\right]$ and that this is equal to the rank of its adjacency matrix

Caste as Community? Networks of social affinity in a South Indian village

We examine three theories of caste and community using new data on social networks among residents of a south Indian village. The first theory treats individual caste groups as separated communities driven by the Brahmanical ideology of hierarchy based on purity and pollution. The second theory departs from the first by placing kings and landlords at the centre of rural (primeval) social structure. Here ritual giving by kings provides the glue that holds a community together by transferring inauspiciousness to gift-recipients and ensuring community welfare. The third theory, that may be treated as a corollary of the second, argues that powerful leaders in the religious and political domains act as patrons of people in their constituencies and forge a sense of community. The resulting community may be single or multi-caste. Using a community structure algorithm from social network analysis, we divide the network of the village into thirteen tight-knit clusters. We find that no cluster or community in the social network has exactly the same boundaries as a caste group in the village. Barring three exceptions, all clusters are multi-caste. Our results are most consistent with the third theory: each cluster has a patron/leader who represents the interests of his constituency at village-level fora and bridges caste and community divides.Social networks, culture, caste, social change, community development, rural India

FROM IRREDUNDANCE TO ANNIHILATION: A BRIEF OVERVIEW OF SOME DOMINATION PARAMETERS OF GRAPHS

Durante los últimos treinta años, el concepto de dominación en grafos ha levantado un interés impresionante. Una bibliografía reciente sobre el tópico contiene más de 1200 referencias y el número de definiciones nuevas está creciendo continuamente. En vez de intentar dar un catálogo de todas ellas, examinamos las nociones más clásicas e importantes (tales como dominación independiente, dominación irredundante, k-cubrimientos, conjuntos k-dominantes, conjuntos Vecindad Perfecta, ...) y algunos de los resultados más significativos.   PALABRAS CLAVES: Teoría de grafos, Dominación.   ABSTRACT During the last thirty years, the concept of domination in graphs has generated an impressive interest. A recent bibliography on the subject contains more than 1200 references and the number of new definitions is continually increasing. Rather than trying to give a catalogue of all of them, we survey the most classical and important notions (as independent domination, irredundant domination, k-coverings, k-dominating sets, Perfect Neighborhood sets, ...) and some of the most significant results.   KEY WORDS: Graph theory, Domination

Introduction to the prisoners and guards game

We study the half-dependent problem for the king graph Kn. We give proofs to establish the values h(Kn) for n ∈ {1, 2, 3, 4, 5, 6} and an upper bound for h(Kn) in general. These proofs are independent of computer assisted results. Also, we introduce a two-player game whose winning strategy is tightly related with the values h(Kn). This strategy is analyzed here for n = 3 and some facts are given for the case n = 4. Although the rules of the game are very simple, the winning strategy is highly complex even for n = 4

Introduction to the Prisoners Versus Guards Game

We introduce a two-player game in which one and his/her opponent attempt to pack as many ``prisoners'' as possible on the squares of an n-by-n checkerboard; each prisoner has to be ``protected'' by at least as many guards as the number of the other prisoners adjacent. Initially, the board is covered entirely with guards. The players take turns adjusting the board configuration using one of the following rules in each turn: I. Replace one guard with a prisoner of the player's color. II. Replace one prisoner of either color with a guard and replace two other guards with prisoners of the player's color. We analyze winning strategies for small n (n<5) and the maximum number of prisoners in general. We show that this maximum is less than (7n^2+4n)/11 and conjecture it is more likely 3n^2/5+O(n).Comment: 15 pages, 1 figur

Investigations in the semi-strong product of graphs and bootstrap percolation

The semi-strong product of graphs G and H is a way of forming a new graph from the graphs G and H. The vertex set of the semi-strong product is the Cartesian product of the vertex sets of G and H, V(G) x V(H). The edges of the semi-strong product are determined as follows: (g1,h1)(g2,h2) is an edge of the product whenever g1g2 is an edge of G and h1h2 is an edge of H or g1 = g2 and h1h2 is an edge of H. A natural subject for investigation is to determine properties of the semi-strong product in terms of those properties of its factors. We investigate distance, independence, matching, and domination in the semi-strong product Bootstrap Percolation is a process defined on a graph. We begin with an initial set of infected vertices. In each subsequent round, uninfected vertices become infected if they are adjacent to at least r infected vertices. Once infected, vertices remain infected. The parameter r is called the percolation threshold. When G is finite, the infection either stops at a proper subset of G or all of V(G) becomes infected. If all of V(G) eventually becomes infected, then we say that the infection percolates and we call the initial set of infected vertices a percolating set. The cardinality of a minimum percolating set of G with percolation threshold r is denoted m(G,r). We determine m(G,r) for certain Kneser graphs and bipartite Kneser graphs