Second-order Form Research Articles

Exact solvability of a two-dimensional real singular Morse potential

The supersymmetric approach in the form of second order intertwining relations is used to prove the exact solvability of two-dimensional Schrodinger equation with generalized two-dimensional Morse potential for $a_0=-1/2$. This two-parametric model is not amenable to conventional separation of variables, but it is completely integrable: the symmetry operator of fourth order in momenta exists. All bound state energies are found explicitly, and all corresponding wave functions are built analytically. By means of shape invariance property, the result is extended to the hierarchy of Morse models with arbitrary integer and half-integer values $a_k=-(k+1)/2.$

On the Ostrogradski instability for higher-order derivative theories and a pseudo-mechanical energy

A two-degree of freedom spring-mass system, described by two coupled second-order ODEs, is considered from a higher-order one-particle (HOOP) viewpoint. A Lagrangian leading to the single fourth-order differential equation now contains time derivatives greater than the first order. The Hamiltonian constructed according to Ostrogradski's method, traditionally regarded as the conserved energy of the system, is negative for one of the modes of vibration, an attribute that would lead to a conclusion in some branches of physics that the mode is unphysical. Reversing this process, a given fourth-order equation, with no apparent underlying second-order structure, is cast into a coupled second-order form allowing one to construct a pseudo-mechanical energy which, unlike the Ostrogradski Hamiltonian, is always positive. The introduction of a viscous damping element leads to velocity and jerk (third derivative)-dependent terms within the HOOP description. In contrast, a physical realisation leading to an isolated velocity-dependent term in the HOOP description shows the instability to be flutter caused by external excitation at the higher natural frequency, rather than an exchange of energy between positive and negative energy modes.

Comment on ‘A note on first-order formalism and odd-derivative actions’ by S Deser

We argue that the obstacles to having a first-order formalism for odd-derivative actions presented in a pedagogical note by Deser (Class. Quantum Grav. 23 5773) are based on examples which are not first-order forms of the original actions. The general derivation of an equivalent first-order form of the original second-order action is illustrated using the example of topologically massive electrodynamics (TME). The correct first-order formulations of the TME model keep intact the gauge invariance presented in its second-order form demonstrating that the gauge invariance present in the original action is not lost when one correctly employs the Ostrogradsky procedure.

A 2-D/3-D cartesian geometry non-conforming spherical harmonic neutron transport solver

A new 2-D/3-D transport core solver for the time-independent Boltzmann transport equation is presented. This solver, named Fiesta, is based on the second-order even-parity form of the transport equation. The angular discretization is performed through the expansion of the angular neutron flux into spherical harmonics ( P N method). The novelty of this solver is the use of non-conforming finite elements for the spatial discretization. Such elements lead to a discontinuous scalar flux approximation. This interface continuity requirement relaxation property is shared with mixed-dual formulations discretized using Raviart–Thomas finite elements. Encouraging numerical results are presented.

OPTIMAL POLICY IN RATIONAL EXPECTATIONS MODELS: NEW SOLUTION ALGORITHMS

This paper develops methods to solve for optimal discretionary policies and optimal commitment policies in rational expectations models. These algorithms, which allow the optimization constraints to be conveniently expressed in second-order structural form, are more general than existing methods and are simple to apply. We use several New Keynesian business cycle models to illustrate their application. Simulations show that the procedures developed in this paper can quickly solve small-scale models and that they can be usefully and effectively applied to medium- and large-scale models.

Developing a Formulation Based upon Curvature for Analysis of Nonprismatic Curved Beams

A new element with three nodal curvatures has been considered for analysis of the nonprismatic curved beams by finite element method. In the formulation developed, the force-curvature relationships in polar coordinate system have been obtained first, then the curvature of the element has been assumed to have a second-order polynomial function form and the radial, tangential displacements, and rotation of the cross section have been found as a function of the curvature accounting for the effects of the cross section variation. Moreover, the relationship between nodal curvatures and nodal deformations has been calculated and used for determining the deformations in terms of curvature at an arbitrary point. The total potential energy has been calculated accounting for bending, shear, and tangential deformations. Invoking the stationary condition of the system, the force-deformation relationship for the element has been obtained. Using this relationship, the stiffness matrix and the equivalent fixed loads applying at the nodes have been computed. The results obtained have been compared with the results of some other references through several numerical examples. The comparison indicates that the present formulation has enough accuracy in analysis of thin and thick nonprismatic curved beams.

L-functions of second-order cusp forms

We discuss equivalent definitions of holomorphic second-order cusp forms and prove bounds on their Fourier coefficients. We also introduce their associated L-functions, prove functional equations for twisted versions of these L-functions and establish a criterion for a Dirichlet series to originate from a second order form. In the last section we investigate the effect of adding an assumption of periodicity to this criterion.

Interior penalty discontinuous Galerkin method for Maxwell's equations: Energy norm error estimates

We develop the symmetric interior penalty discontinuous Galerkin (DG) method for the time-dependent Maxwell equations in second-order form. We derive optimal a priori error estimates in the energy norm for smooth solutions. We also consider the case of low-regularity solutions that have singularities in space.

Reconstruction in optical tomography using the PN approximations

In this paper, we consider the inverse problem of reconstructing the absorption and scattering coefficients of the radiative transfer equation (RTE) from measurements of photon current transmitted through a scattering medium in the frequency domain. We consider an output least-squares formulation of this problem and derive the appropriate forward operators and their Fréchet derivatives. For efficient implementation, we use the second-order form of the RTE and discuss its solution using a finite element method. The PN approximation is used to expand the radiance in spherical harmonics, which leads to a large sparse matrix system that can be efficiently solved. Examples are shown in the low-scattering case where the diffusion approximation fails.

Suppression of limit cycles in second-order companion form digital filters with saturation arithmetic

A condition for second-order companion form digital filters with time variant nondeterministic saturation overflow arithmetic to be free of limit cycles was previously given by Ooba. The condition corresponds to a region which is a subset of the stability triangle. In the present paper, time invariant deterministic saturation nonlinearities are considered. It is shown that, with such nonlinearities, the system is free of limit cycles in whole of the stability triangle.

Discrete boundary treatment for the shifted wave equation in second-order form and related problems

We present strongly stable semi-discrete finite difference approximations to the quarter space problem (x > 0, t > 0) for the first order in time, second order in space wave equation with a shift term. We consider space-like (pure outflow) and time-like boundaries, with either second- or fourth-order accuracy. These discrete boundary conditions suggest a general prescription for boundary conditions in finite difference codes approximating first order in time, second order in space hyperbolic problems, such as those that appear in numerical relativity. As an example we construct boundary conditions for the Nagy–Ortiz–Reula formulation of the Einstein equations coupled to a scalar field in spherical symmetry.

Problems which are well posed in a generalized sense with applications to the Einstein equations

In the harmonic description of general relativity, the principal part of the Einstein equations reduces to a constrained system of ten curved space wave equations for the components of the spacetime metric. We use the pseudo- differential theory of systems which are strongly well posed in the generalized sense to establish the well posedness of constraint-preserving boundary conditions for this system when treated in a second-order differential form. The boundary conditions are of a generalized Sommerfeld type that is benevolent for numerical calculation.

Unstructured lattice Boltzmann equation with memory

Starting from a second-order differential form of the semi-discrete Boltzmann equation, we construct a new finite-volume lattice Boltzmann equation on unstructured grids (ULBE). The new scheme (ULBE with memory) is demonstrated for the case of a Taylor-vortex flow and shown to produce stable and accurate results with time-step more than an order of magnitude above the standard LBE stability threshold.

A non-conforming generalization of Raviart–Thomas elements to the spherical harmonic form of the even-parity neutron transport equation

The Raviart–Thomas finite elements provide an appropriate spatial discretization of the mixed-dual form of the diffusion equation. This discretization can then be coupled to an efficient solution method. The high performances achieved by such an approach triggered research on its possible generalization to the transport equation using a spherical harmonic (or P N) angular approximation. In view of the difficulty of developing a straightforward generalization within the mixed-dual framework, we here consider 2D non-conforming (i.e., allowing interface discontinuities) finite elements coupled to the second-order form of the transport equation. This non-conforming approach keeps the mixed-dual property of the relaxation of the flux interface continuity constraint. We investigate different non-conforming elements and compare them to the well-known Lagrangian conforming elements.

A Discrete Quasi-Coordinate Formulation for the Dynamics of Elastic Bodies

The discretized equations of motion for elastic systems are typically displayed in second-order form. That is, the elastic displacements are represented by a set of discretized (generalized) coordinates, such as those used in a finite-element method, and the elastic rates are simply taken to be the time-derivatives of these displacements. Unfortunately, this approach leads to unpleasant and computationally intensive inertial terms when rigid rotations of a body must be taken into account, as is so often the case in multibody dynamics. An alternative approach, presented here, assumes the elastic rates to be discretized independently of the elastic displacements. The resulting dynamical equations of motion are simplified in form, and the computational cost is correspondingly lessened. However, a slightly more complex kinematical relation between the rate coordinates and the displacement coordinates is required. This tack leads to what may be described as a discrete quasi-coordinate formulation.

A converse theorem for second-order modular forms of level N

A converse theorem for second-order modular forms of level N

Spatial discretizations for self-adjoint forms of the radiative transfer equations

There are three commonly recognized second-order self-adjoint forms of the neutron transport equation: the even-parity equations, the odd-parity equations, and the self-adjoint angular flux equations. Because all of these equations contain second-order spatial derivatives and are self-adjoint for the mono-energetic case, standard continuous finite-element discretization techniques have proved quite effective when applied to the spatial variables. We first derive analogs of these equations for the case of time-dependent radiative transfer. The primary unknowns for these equations are functions of the angular intensity rather than the angular flux, hence the analog of the self-adjoint angular flux equation is referred to as the self-adjoint angular intensity equation. Then we describe a general, arbitrary-order, continuous spatial finite-element approach that is applied to each of the three equations in conjunction with backward-Euler differencing in time. We refer to it as the “standard” technique. We also introduce an alternative spatial discretization scheme for the self-adjoint angular intensity equation that requires far fewer unknowns than the standard method, but appears to give comparable accuracy. Computational results are given that demonstrate the validity of both of these discretization schemes.

Optimal Policy Rules in Rational-Expectations Models: New Solution Algorithms

This paper develops methods to solve for optimal discretionary policies and optimal commitment policies in rational expectations models. These algorithms, which allow the optimization constraints to be conveniently expressed in second-order structural form, are more general than existing methods and are simple to apply. We use several New Keynesian business cycle models to illustrate their application. Simulations show that the procedures developed in this paper can quickly solve small-scale models and that they can be usefully and effectively applied to medium- and large-scale models.

Ab initio density functional theory: The best of both worlds?

Density functional theory (DFT), in its current local, gradient corrected, and hybrid implementations and their extensions, is approaching an impasse. To continue to progress toward the quality of results demanded by today's ab initio quantum chemistry encourages a new direction. We believe ab initio DFT is a promising route to pursue. Whereas conventional DFT cannot describe weak interactions, photoelectron spectra, or many potential energy surfaces, ab initio DFT, even in its initial, optimized effective potential, second-order many-body perturbation theory form [OEP (2)-semi canonical], is shown to do all well. In fact, we obtain accuracy that frequently exceeds MP2, being competitive with coupled-cluster theory in some cases. Furthermore, this is accomplished within a relatively fast computational procedure that scales like iterative second order. We illustrate our results with several molecular examples including Ne2,Be2,F2, and benzene.

The analysis of irregularly observed stochastic astronomical time-series - I. Basics of linear stochastic differential equations

The theory of low-order linear stochastic differential equations is reviewed. Solutions to these equations give the continuous time analogues of discrete time autoregressive time-series. Explicit forms for the power spectra and covariance functions of first- and second-order forms are given. A conceptually simple method is described for fitting continuous time autoregressive models to data. Formulae giving the standard errors of the parameter estimates are derived. Simulated data are used to verify the performance of the methods. Irregularly spaced observations of the two hydrogen-deficient stars FQ Aqr and NO Ser are analysed. In the case of FQ Aqr the best-fitting model is of second order, and describes a quasi-periodicity of about 20 d with an e-folding time of 3.7 d. The NO Ser data are best fitted by a first-order model with an e-folding time of 7.2 d.