Iranian Journal of science and Technology (A: Siences)
Volume:34 Issue: 2, Spring 2010

In this paper lightlike ruled surfaces in 3 1=(3,-dx2+dy2+dz2) are studied with respect to whether ruling curves are spacelike or null. It is seen that, in the first case the Gaussian curvature of the ruled surfaces vanishes. In the second case the Gaussian curvature of the ruled surfaces are negative. In the second case lightlike ruled surfaces are totally umbilical. Furthermore, lightlike surfaces of revolution are shown to be only cones, and the second type lightlike ruled surface.

In this paper, the Goursat problem of a third order equation on cases of explicit solvability is investigated, with the help of the Riemann function. Some results and one theorem are given concerning the existence and uniqueness for the solution of the suggested problem.

Reduced differential transform method (RDTM) is applied to various wave equations. To assess the accuracy of the solutions, we compare the results with the exact solutions and variational iteration method.The results reveal that the RDTM is very effective, convenient and quite accurate to systems of nonlinear equations.

Exact solution for steady two-dimensional flow of an incompressible magma is obtained. The magmatic flow is studied by considering the magma as a second grade fluid. The governing partial differential equations are transformed to ordinary differential equations by symmetry transformations. Results are discussed through graphs to understand the rheology of the flowing magma.

The electronic structure, energy band structure and electronic density of 2 SnO ceramic in cubic phase have been investigated using first principle full potential-linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). Local density approximation (LDA) and the generalized gradient approximation (GGA), which are based on exchange- correlation energy optimization were used. The band gap was 2.2 eV at point in the Brillouin zone within our approach. Calculations of the band structure and electronic structure of 2 SnO were in a good agreement with the previous experimental and theoretical results with different approximations. Moreover, electronic density map shows that the bonding between Sn and O atoms is ionic.

The problem of discrimination between two stationary AR(p) plus noise processes is considered when the noise process are different in two models. The discrimination rule leads to a quadratic form with cumbersome matrices. An approximate and analytic form is given to distribution of the discriminant. The simulation study has been used to show the performance of discrimination rule. The cumulants of discriminant function are obtained and show them to be very close to the true values given in literature.

A notion of amenability for topological semigroups is introduced. A topological semigroup S is called Johnson amenable if for every Banach S -bimodule E, every bounded crossed homomorphism from S to E* is principal. In this paper it is shown that a discrete semigroup S is Johnson amenable if and only if 1(S) is an amenable Banach algebra. Also, we show that if a topological semigroup S is Johnson amenable, then it is amenable, but the converse is not true.

This paper presents an efficient modification of the variational iteration method for solvingboundary value problems using the chebyshev polynomials. The proposed method can be applied to linear and nonlinear models. The scheme is tested for some examples and the obtained results demonstrate the reliability and efficiency of the proposed method.