An exact solution to the Dirac equation for a time dependent Hamiltonian in 1-1D space-time

We find an exact solution to the Dirac equation in 1-1 dimensional space-time in the presence of a time-dependent potential which consists of a combination of electric, scalar, and pseudoscalar terms.Comment: Five page

Graphical representation and generalization in sequences problems

In this paper we present different ways used by Secondary students to generalize when they try to solve problems involving sequences. 359 Spanish students solved generalization problems in a written test. These problems were posed through particular terms expressed in different representations. We present examples that illustrate different ways of achieving various types of generalization and how students express generalization. We identify graphical representation of generalization as a useful tool of getting other ways of expressing generalization, and we analyze its connection with other ways of expressing it

Comment on "Wave functions for a Duffin-Kemmer-Petiau particle in a time-dependent potential"

It is shown that the paper "Wave functions for a Duffin-Kemmer-Petiau particle in a time-dependent potential", by Merad and Bensaid [J. Math. Phys. 48, 073515 (2007)] is not correct in using inadvertently a non-Hermitian Hamiltonian in a formalism that does require Hermitian Hamiltonians.Comment: 2 page

On the Dirac equation with PT-symmetric potentials in the presence of position-dependent mass

The relativistic problem of fermions subject to a PT-symmetric potential in the presence of position-dependent mass is reinvestigated. The influence of the PT-symmetric potential in the continuity equation and in the orthonormalization condition are analyzed. In addition, a misconception diffused in the literature on the interaction of neutral fermions is clarified.Comment: 8 page

The role of Central American Trade in developing El Salvador’s high-value exports

This paper describes the development and evolution of El Salvador’s industrial exports. This paper found that El Salvador’s exports to other Central American countries enhance business opportunities observed in an increase in the number of new products exported to this region and this products are more complex compared to the regular exports outside region. However, this paper also finds that the products that are exported first to Central America and then to the rest of the world are not the most sophisticated. El Salvador may benefit from trade diversion that prevents it from exporting high quality products to the world except Central America.Titulo original en español: Evolución y desarrollo de la complejidad de las exportaciones del sector industrial salvadoreño en los últimos 20 año

The String Uncertainty Relations follow from the New Relativity Principle

The String Uncertainty Relations have been known for some time as the stringy corrections to the original Heisenberg's Uncertainty principle. In this letter the Stringy Uncertainty relations, and corrections thereof, are explicitly derived from the New Relativity Principle that treats all dimensions and signatures on the same footing and which is based on the postulate that the Planck scale is the minimal length in Nature in the same vein that the speed of light was taken as the maximum velocity in Einstein's theory of Special Relativity. The Regge behaviour of the string's spectrum is also a natural consequence of this New Relativity Principle.Comment: Revised Tex file. 5 pages. Important change of notation for the polymomenta coordinates has been made instead of using misleading wedge products. The Cosmological constant term is include