Chain hexagonal cacti: Matchings and independent sets

In this paper we consider three classes of chain hexagonal cacti and study their matching and independence related properties. Explicit recurrences are derived for their matching and independence polynomials, and explicit formulae are presented for the number of matchings and independents sets of certain types. Bivariate generating functions for the number of matchings and independent sets of certain types are also computed and then used to deduce the expected size of matchings and independent sets in chains of given length. It is shown that the extremal chain hexagonal cacti with respect to the number of matchings and of independent sets belong to one of the considered types. Possible directions of further research are discussed.     

Triangle-free graphs whose independence number equals the degree

In a triangle-free graph, the neighbourhood of every vertex is an independent set. We investigate the class S of triangle-free graphs where the neighbourhoods of vertices are maximum independent sets. Such a graph G must be regular of degree d=α(G) and the fractional chromatic number must satisfy χ_f(G)=|G|/α(G). We indicate that S is a rich family of graphs by determining the rational numbers c for which there is a graph G∈S with χ_f(G)=c except for a small gap, where we cannot prove the full statement. The statements for c≥3 are obtained by using, modifying, and re-analysing constructions of Sidorenko, Mycielski, and Bauer, van den Heuvel and Schmeichel, while the case c<3 is settled by a recent result of Brandt and Thomassé. We will also investigate the relation between other parameters of certain graphs in S like chromatic number and toughness.     

Determining classes and independence

A class of functions of a random element X is sufficiently rich to determine the distribution of X if and only if it is sufficiently rich to determine whether X is independent of other random functions.     

Stress state of a hollow round cylinder with side holes under different pressure and temperature impact

The stress–strain state of a hollow cylinder (pipe) with lateral holes is considered. The pipe is under the action of internal and external hydrostatic pressures P₁ and P₂ and also a temperature effect. Such a pipe is used in oil–gas recovery, rocket engineering, chemical machinery industry and etc. There is no detailed analytic solution of this problem in literature to date. The number of holes through which a liquid or gas can flow into the pipe interior (and vice versa) depends on the loads, the size of the cross-section and the material of the pipe. The proper selection of the arrangement of holes and their diameters is determined from an analytic solution. In addition, the maximum boundary loads (of pressures) at which the pipe cracks is also found.     

MAXIMUM GENUS, INDEPENDENCE NUMBER AND GIRTH

1.IntroductionThegraphconsideredhereisconnectedandsimpleunlessstatedotherwise.Forterminologyandnotationwithoutexplanation,wereferto[l].Inthispaperwestudythemaximumgenusofagraph,aninvariancethatcharacterizesthegraphcellularlyembeddedinanorielltablesurface.Recallthatthemaximumgenussdenotedby7M(G),ofthegraphGisthemaximumintegernumberkwiththepropertythatthereexistsacellularembeddingofGontheorientablesurfaceSofgenusk.TheEulerpolyhedralequationshowsthatthemaximumgenusofanygraphGsatisfiesthefollo…     

On the validity of the independence principle applied to the vortex-induced vibrations of a flexible cylinder inclined at 60°

The vortex-induced vibrations (VIV) of a flexible cylinder inclined at 60° are investigated by means of direct numerical simulation, at a Reynolds number equal to 500, based on the cylinder diameter and inflow velocity. The cylinder has a circular cross-section and a length to diameter aspect ratio equal to 50; it is modeled as a tension-dominated structure which is free to oscillate in the in-line and cross-flow directions. The behavior of the coupled fluid–structure system is examined for two values of the tension. Particular attention is paid to the validity of the independence principle (IP) which states that the inclined and normal-incidence body cases are comparable if the inflow velocity normal component is used to scale the physical quantities.The flexible cylinder exhibits regular VIV for both values of the tension. In the high-tension configuration, where the in-line bending of the structure remains small, the IP is shown to be valid for the prediction of the cylinder responses and the fluid forces. In contrast, in the lower-tension configuration, the behavior of the fluid–structure system deviates from the IP. It is shown that this deviation is connected to the larger in-line bending of the structure which leads to considerably different profiles of the flow velocity locally perpendicular to the body in the inclined and normal cylinder cases. Since the system behavior appears to be mainly driven by this component of the flow, the profile modification induced by the larger in-line bending results in distinct responses: multi-frequency vibrations are observed in the inclined cylinder case whereas mono-frequency oscillations of larger amplitudes develop at normal incidence.     

Validation of association

Recognizing, quantifying and visualizing associations between two variables is increasingly important. This paper investigates how a new function-valued measure of dependence, the quantile dependence function, can be used to construct tests for independence and to provide an easily interpretable diagnostic plot of existing departures from the null model. The dependence function is designed to detect general dependence structure between variables in quantiles of the joint distribution. It gives an insight into how the dependence structure changes in different parts of the joint distribution. We define new estimators of the dependence function, discuss some of their properties, and apply them to construct new tests of independence. Numerical evidence is given to the tests benefits against three recognized independence tests introduced in the previous years. In real-data analysis, we offer the use of our tests and the graphical presentation of the underlying dependence structure.     

On trees with real-rooted independence polynomial

The independence polynomial of a graph $G$ is

$I(G,x)=\sum _{k\ge 0}{i}_k\left(G\right)x^k,$

where $i_k\left(G\right)$ denotes the number of independent sets of $G$ of size $k$ (note that $i_0\left(G\right)=1$). In this paper we show a new method to prove real-rootedness of the independence polynomials of certain families of trees.In particular we will give a new proof of the real-rootedness of the independence polynomials of centipedes (Zhu’s theorem), caterpillars (Wang and Zhu’s theorem), and we will prove a conjecture of Galvin and Hilyard about the real-rootedness of the independence polynomial of the so-called Fibonacci trees.     

On 3-stable number conditions in n-connected claw-free graphs

For a subgraph $X$ of $G$, let ${\alpha }_G^3\left(X\right)$ be the maximum number of vertices of $X$ that are pairwise distance at least three in $G$. In this paper, we prove three theorems. Let $n$ be a positive integer, and let $H$ be a subgraph of an $n$-connected claw-free graph $G$. We prove that if $n\ge 2$, then either $H$ can be covered by a cycle in $G$, or there exists a cycle $C$ in $G$ such that ${\alpha }_G^3(H?V\left(C\right))\le {\alpha }_G^3\left(H\right)?n$. This result generalizes the result of Broersma and Lu that $G$ has a cycle covering all the vertices of $H$ if ${\alpha }_G^3\left(H\right)\le n$. We also prove that if $n\ge 1$, then either $H$ can be covered by a path in $G$, or there exists a path $P$ in $G$ such that ${\alpha }_G^3(H?V\left(P\right))\le {\alpha }_G^3\left(H\right)?n?1$. By using the second result, we prove the third result. For a tree $T$, a vertex of $T$ with degree one is called a leaf of $T$. For an integer $k\ge 2$, a tree which has at most $k$ leaves is called a $k$-ended tree. We prove that if ${\alpha }_G^3\left(H\right)\le n+k?1$, then $G$ has a $k$-ended tree covering all the vertices of $H$. This result gives a positive answer to the conjecture proposed by Kano et al. (2012).