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One way of improving the behavior of finite element schemes for classical, time-dependent Maxwell's equations, is to render them from their hyperbolic character to elliptic form. This paper is devoted to the study of the stabilized linear finite element method for the time harmonic Maxwell's equations in a dual form obtained through the Laplace transformation in time. The model problem is for the particular case of the dielectric permittivity function which is assumed to be constant in a boundary neighborhood. For the stabilized model a coercivity relation is derived that guarantee's the existence of a unique solution for the iscrete problem. The convergence is addressed both in a priori and a posteriori settings. In the a priori error estimates we confirm the theoretical convergence of the scheme in a L2-based, gradient dependent, triple norm. The order of convergence is O(h) in weighted Sobolev space and hence optimal. We also derive, similar, optimal a posteriori error estimates controlled by a certain, weighted, norm of the residual of the computed solution. The posteriori approach is used for constructing adaptive algorithms for the computational purposes. Further, assuming a sufficiently regular solution for the dual problem, we reach the same convergence of O(h). Finally, through implementing several numerical examples, we validate the robustness of the proposed scheme.

We derive a priori error estimates for the standard Galerkin and streamline diffusion finite element methods for the Fermi pencil-beam equation obtained from a fully three dimensional Fokker-Planck equation in space ${\mathbf x}=(x,y,z)$ and velocity $\tilde {\mathbf v}=(\mu, \eta, \xi)$ variables. The Fokker-Planck term appears as a Laplace-Beltrami operator in the unit sphere. The diffusion term in the Fermi equation is obtained as a projection of the FP operator onto the tangent plane to the unit sphere at the pole $(1,0,0)$ and in the direction of $ {\mathbf v}_0=(1,\eta, \xi)$. Hence the Fermi equation, stated in three dimensional spatial domain ${\mathbf x}=(x,y,z)$, depends only on two velocity variables ${\mathbf v}=(\eta, \xi)$. Since, for a certain number of cross-sections, there is a closed form analytic solution available for the Fermi equation, hence an a posteriori error estimate procedure is unnecessary and in our adaptive algorithm for local mesh refinements we employ the a priori approach. Different numerical examples, in two space dimensions are justifying the theoretical results. Implementations show significant reduction of the computational error by using our adaptive algorithm., Comment: 26 pages

We investigate the phase diagram of an anisotropic ferrimagnet spin-(1/2, 1) in the presence of a non-commuting (transverse) magnetic field. We find a magnetization plateau for the isotropic case while there is no plateau for the anisotropic ferrimagnet. The magnetization plateau can appear only when the Hamiltonian has the U(1) symmetry in the presence of the magnetic field. The anisotropic model is driven by the magnetic field from the N\'{e}el phase for low fields to the spin-flop phase for intermediate fields and then to the paramagnetic phase for high fields. We find the quantum critical points and their dependence on the anisotropy of the aforementioned field-induced quantum phase transitions. The spin-flop phase corresponds to the spontaneous breaking of Z2 symmetry. We use the numerical density matrix renormalization group and analytic spin wave theory to find the phase diagram of the model. The energy gap, sublattice magnetization, and total magnetization parallel and perpendicular to the magnetic field are also calculated. The elementary excitation spectrums of the model are obtained via the spin wave theory in the three different regimes depending on the strength of the magnetic field., Comment: 14 pages, 11 eps figures