1,721,129 research outputs found
Remarks on the numerical solution of certain linear complementarity problems
No full textAbstractIn a recent paper, J.K. Aitchison and N.K. Upton have proposed a mathematical model of the behaviour of a cloud formed immediately after the sudden release of a pollutant, together with an algorithm for determining numerical solutions of the resulting system of constrained nonlinear equations and complementarity relations. This algorithm requires, at each step, the solution of a special linear complementarity problem, which is solved by an iterative method. In this note, it is argued that the robustness and reliability of the solution procedure can be improved by the use of standard linear programming techniques
Solving cubic matrix equations arising in conservative dynamics
Full text linkIn this paper we consider the spatial semi-discretization of conservative PDEs. Such finite dimensional approximations of infinite dimensional dynamical systems can be described as flows in suitable matrix spaces, which in turn leads to the need to solve polynomial matrix equations, a classical and important topic both in theoretical and in applied mathematics. Solving numerically these equations is challenging due to the presence of several conservation laws which our finite models incorporate and which must be retained while integrating the equations of motion. In the last thirty years, the theory of geometric integration has provided a variety of techniques to tackle this problem. These numerical methods require solving both direct and inverse problems in matrix spaces. We present three algorithms to solve a cubic matrix equation arising in the geometric integration of isospectral flows. This type of ODEs includes finite models of ideal hydrodynamics, plasma dynamics, and spin particles, which we use as test problems for our algorithms.In this paper we consider the spatial semi-discretization of conservative PDEs. Such finite dimensional approximations of infinite dimensional dynamical systems can be described as flows in suitable matrix spaces, which in turn leads to the need to solve polynomial matrix equations, a classical and important topic both in theoretical and in applied mathematics. Solving numerically these equations is challenging due to the presence of several conservation laws which our finite models incorporate and which must be retained while integrating the equations of motion. In the last thirty years, the theory of geometric integration has provided a variety of techniques to tackle this problem. These numerical methods require solving both direct and inverse problems in matrix spaces. We present three algorithms to solve a cubic matrix equation arising in the geometric integration of isospectral flows. This type of ODEs includes finite models of ideal hydrodynamics, plasma dynamics, and spin parti..
Defeasible Arguments and Context Dependence
No full textDefeasibility is a central concept in non-monotonic reasoning. In this paper we connect the peculiar looseness of non-strictly-deductive inference – looseness grounded on “default” or defeasible assumptions – to the looseness of referential uses of definite descriptions. In both cases we are faced with the problem of revising some premises of an argument and change the conclusion. We aim to show that inferential defeasibility requires attention to lexical and cultural context dependence and to different forms of arguments, also depending on whether premises are given as general propositions or singular propositions. Although our main point is on different aspects of defeasible arguments, the paper is also an attempt to link problems typically discussed in the philosophy of science and problems discussed in the philosophy of language
Robust approximate inverse preconditioning for the conjugate gradient method
No full textWe present a variant of the AINV factorized sparse approximate inverse algorithm which is applicable to any symmetric positive definite matrix. The new preconditioner is breakdown-free and, when used in conjunction with the conjugate gradient method, results in a reliable solver for highly ill-conditioned linear systems. We also investigate an alternative approach to a stable approximate inverse algorithm, based on the idea of diagonally compensated reduction of matrix entries. The results of numerical tests on challenging linear systems arising from finite element modeling of elasticity and diffusion problems are presented
HPLC-DAD-MSn to investigate the photodegradation pathway of nicosulfuron in aqueous solution
No full textThe environmental interest of sulfonylurea herbicides was derived from the possibility of diffusion and penetration of these herbicides in the deepest layers of the ground, in particular in sandy or clay-poor soils, up to the ground waters; another interest of the study is their natural degradation pathway which leads to the formation of new species that are potentially more toxic and stable than the precursor herbicides. In this case, a lower persistence in the environment unfortunately does not correspond to a lower toxicity: hence, the importance of the identification of the species can be potentially formed. Here, nicosulfuron, a typical sulfonylurea herbicide, is considered in order to outline the environmental fate of the molecules generating from the simulation of one of the natural processes that can occur, i.e. photoinduced degradation. Aqueous nicosolfuron solutions underwent a simulated sun irradiation: the new species formed during the degradation process were identified by HPLC-DAD-MS/MS and a degradation pathway was proposed. The effect of temperature and the contribution of the hydrolysis were also evaluated. The use of ESI in both positive ion (PI) and negative ion (NI) mode and APCI in PI mode permits to obtain integrated information about the transformation products that can form; moreover, a study of the total ion chromatogram followed by the extraction of the SIM chromatograms of the most intense m/z signals made possible the identification of five possible photodegradation transformation products
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