Scielo RSS <![CDATA[Proyecciones (Antofagasta)]]> vol. 31 num. 3 lang. en <![CDATA[SciELO Logo]]> <![CDATA[A note on rescalings of the skew-normal distribution]]> In this article, we show that certain skew-normal parametric statistical models are a result of rescalings of the skew normal model studied by Azzalini (1985). Using this procedure we define a class of skew-normal distributions and we study its moment, skewness and kurtosis coefficients. At the end of this article we will use this class of distribution to make some extensions of the skew-normal model. <![CDATA[<b>Bounded linear operators for some new matrix</b> <b>transformations</b>]]> In this paper, we define (ó, 0)-convergence and characterize (ó, è)-conservative, (ó, 0)-regular, (ó, 0)-coercive matrices and we also determine the associated bounded linear operators for these matrix classes. <![CDATA[<b>Optimal replacement in a system of <strong>n</strong>-machines with random horizon</b>]]> This paper considers a system consisting of independently operating n-machines, which follows a deterioration processes with an associated cost function. It is assumed that the system is observed at discrete time and the objective function is the total expected cost. Also, it is considered that the horizon of the problem is random. For this problem, a replacement optimal policy that minimize the operation cost of the system is provided. Besides, a numerical example through a program in Matlab is presented. <![CDATA[<b>On (i, j)-</b><b>ù</b><b>-preopen sets</b>]]> In this paper, we introduce and study the notion of (i, j)-u!-preopen sets as a generalization of (i, j)-preopen sets in bitopological space. <![CDATA[<b>On the spectral radius of weighted digraphs</b>]]> We consider the weighted digraphs in which the arc weights are positive definite matrices. We obtain some upper bounds for the spectral radius of these digraphs and characterize the digraphs achieving the upper bounds. Some known upper bounds are then special cases of our results. <![CDATA[<b>Symmetric regular cacti-properties and enumeration</b>]]> A cactus is a connected graph whose all the blocks are isomorphic to cycle or complete graph on n vertices. We introduce symmetric regular cacti and a procedure for their construction. We discuss some characteristics of symmetric regular cacti. The number of symmetric regular cacti on given number of vertices are also enumerated. A possible application of present work to a real world problem for the committee formation is also suggested. <![CDATA[<b>New numerical radius inequalities for certain operator matrices</b>]]> In this paper we prove some upper and lower bounds for the numerical radius of the off-diagonal part of 3 X 3 operator matrices and some bounds for the numerical radius inequalities of the general 3 X 3 operator matrix. <![CDATA[<b>Double lacunary sequence spaces of double sequence of interval numbers</b>]]> In this paper we introduce the concepts of double lacunary strongly convergence and double lacunary statistical convergence of double interval numbers. We prove some inclusion relations and study some of their properties.