SHAHID CHAMRAN UNIVERSITY JOURNAL OF SCIENCE
Year:2008 | Volume:NEW SERIES | Issue:18 (SECTION A)|Papers Count:10

In this article, the subject "z-ideals" with 50 years oldness is reviewed. We study the evolution of z-ideals in any ring and specially in the ring of continuous functions C(X) in which z-ideal is introduced. All properties of z-ideals and the relations between z-ideals and prime ideals in C(X) are propounded and finally z-ideals are known as connecting bridge between topological properties of the space X and algebraic properties of the ring C(X) by several facts and results.

General vaccination programmers aim to reduce the spread out of infectious diseases. In medical studies, the disease reproduction means that the average number of disease infections, is essential and is usually tried to be below the unity. If it is one or greater, it means disease outbreak and the revaccination is required. In this paper, we describe a method for monitoring the value of R using surveillance data. The model is based on branching process in which R, is identified with the offspring mean. We derive unconditional likelihood for the offspring mean using data on outbreak size when distributions of offspring mean are Poisson or geometric to estimate the outbreak rate of measles disease in Fars province.

Nonsampling errors can occur in survey implementation. Response error is an important part of nonsampling error, and modeling of the error has received a particular attention over the years. In this paper, a model of response error in face to face surveys is considered. For the model, test statistics of the effects are determined based on performing analysis of variance table. Furthermore, restricted maximum likelihood (REML) method is considered and REML estimators of variance components (in two cases) are computed. As an application of the presented results, REML estimates are calculated for some data sets. Then, nonlinear programming is applied to assess the obtained estimates. These computations confirm the presented REML estimators.

In spatial analysis, the variogram function that determines the spatial correlation structure of the data is usually unknown and it should be estimated by observations. Although there are several estimation methods for variogram parameters, limited number of observations produces in much uncertainty in variogram parameter estimates and finally non-precision of the spatial prediction. In this paper, precision measures of the weighted least squares estimates of variogram parameters are determined by separate block bootstrap method. Next, these estimates are corrected using precision measures. Then, it is shown by cross-validation method that using a variogram with corrected parameter estimates with result in increasing spatial prediction precision.

In this article we will observe that Cm(X), the space C(X) with m - topology, is a regular Hausdorff space and we show that Cm(X) is first countable iff X is pseudo compact. We prove that Cm(X) ,is never a P - space (or pseudo compact). It is shown that the collection of zero divisor functions is closed in Cm(X) iff X is an almost P -space. The family of units in Cm(X) is investigated and using this, we observe that Cm(X) is never an extremely disconnected space. Finally, we show that C¥(X) is closed in Cm,(X) and the closure of Ck(X) is characterized in terms of maximal ideals of C(X).

The multiplicatively closed set is an important and useful concept in algebra. However, the class of these subsets of a ring has not been considered as an algebraic structure. In this paper, an algebraic structure on this class will be defined and the properties of this structure will be presented. In particular, we will study some conditions under which this algebraic structure becomes a copy of the Boolean ring P(X).

In this paper, first we introduce second-order cone programming. Then we convert some classes of the fractional programming problems into the problems of the type of the second-order cone programming and apply potential reduction interior point method to solve it. By solving this class of problems, optimal solution of the original problem is obtained.

Due to the importance of GaN as a wide band gap semiconductor in electronic and optoelectronic devices the study of electrical transport properties of this material has been of interested to many researchers. In general, the presence of impurities and dislocations affect the intrinsic properties of semiconductors. Here our goal is a theoretical study for the effect of different n-type dopants (Si, Ge and O) on electrical properties in three samples of this material. Our calculations are based on charge neutrality and electron scattering mechanisms based on relaxation time approximation. The results of our analysis indicate that Si is an appropriate dopant with shallow energy level (17 meV), relative to the conduction band edge, whose presence leads to a relatively low dislocation density of about 109 cm-2. In contrast, Ge and O correspond to a deeper impurity level of 19 and 28 meV, respectively. Also, the presence of such impurities leads to a higher dislocation and nitrogen vacancy concentrations of about 103and 3 in this material.

Self-inductance coefficient is one of the most important parameters in electromagnetic forming process. Presence of work piece near coil, in electromagnetic forming process, affects the coil self-inductance coefficient seriously and therefore other process parameters such as electrical current amplitude, frequency and pressure applied on the work piece are affected too. In this paper, the coil work piece equivalent self-inductance coefficient, which has not been calculated in previous relevant works, is calculated numerically using a commercial FE code. To obtain the electrical current distribution profile in the coil cross sections, first a cylindrical coil is simulated without any work piece. Then its self-inductance coefficient is compared with analytical calculation results. By using simulation results, the variation of magnetic-flow versus electrical-current has been plotted. The curve slope (self-inductance coefficient) has been extracted by using least squares method. The calculated self-inductance coefficients of the coils with various dimensions show a good agreement with analytical results presented in other references. Next, this method is used to simulate and calculate the coil-work piece equivalent self-inductance coefficient in electromagnetic tube expansion process. At the end, as a parametric study, work pieces with various diameters are simulated and their equivalent self-inductance coefficients are calculated. The results agree with the experimental results due to the presence of an external conductive case around a cylindrical coil, which has already been presented in the literature.