The numerical analysis of variational integrators relies on variational error analysis, which relates the order of accuracy of a variational integrator with the order of approximation of the exact discrete Lagrangian by a computable discrete Lagrangian. The exact discrete Lagrangian can either be characterized variationally, or in terms of Jacobi's solution of the Hamilton-Jacobi equation. These two characterizations lead to the Galerkin and shooting-based constructions for discrete Lagrangians, which depend on a choice of a numerical quadrature formula, together with either a finite-dimensional function space or a one-step method. We prove that the properties of the quadrature formula, finite-dimensional function space, and underlying one-step method determine the order of accuracy and momentum-conservation properties of the associated variational integrators. We also illustrate these systematic methods for constructing variational integrators with numerical examples.

We introduce a novel technique for constructing higher-order variational integrators for Hamiltonian systems of ordinary differential equations. In the construction of the discrete Lagrangian we adopt Hermite interpolation polynomials and the Euler-Maclaurin quadrature formula and apply collocation to the Euler-Lagrange equation and its prolongation. Considerable attention is devoted to the order analysis of the resulting variational integrators in terms of approximation properties of the Hermite polynomials and quadrature errors. In particular, the order of the variational integrator can be computed a priori based on the quadrature error estimate. The analysis in the paper is straightforward compared to the order theory for Runge-Kutta methods. Finally, a performance comparison is presented on a selection of these integrators.

The problem of finding the correct asymptotic rate of approximation by polynomial loops in dependence of the smoothness of the elements of a loop group seems not well-understood in general. For matrix Lie groups such as SU(N ), it can be viewed as a problem of nonlinearly constrained trigonometric approximation. Motivated by applications to optical FIR filter design and control, we present some initial results for the case of SU(N )-loops, N ≥ 2. In particular, using representations via the exponential map and first order splitting methods, we prove that the best approximation of an SU(N )-loop belonging to a Hölder-Zygmund class Lip α , α > 1/2, by a polynomial SU(N )-loop of degree ≤ n is of the order O(n −α/(1+α) ) as n → ∞. Although this approximation rate is not considered final, to our knowledge it is the first general, nontrivial result of this type.

The problem of constructing smooth interpolating curves in non-Euclidean spaces finds applications in different areas of science. In this paper we propose a scheme to generate interpolating curves in Lie groups, focusing on a special orthogonal group SO(n). Our technique is based on the exponential representation of elements of the group, which allows to transfer the problem to the corresponding Lie algebra so (n) and benefit from the linearity of this space. Due to the exponential representation we can obtain a high degree of smoothness of an interpolating curve at relatively low costs. The underlying problem is challenging because the standard SO(n) −→ so(n) map is multi-valued.

Abstract. In this paper, we present a variational integrator that is based on an approximation of the Euler-Lagrange boundary-value problem via Taylor's method. This can viewed as a special case of the shooting-based variational integrator introduced in [11]. The Taylor variational integrator exploits the structure of the Taylor method, which results in a shooting method that is one order higher compared to other shooting methods based on a one-step method of the same order. In addition, this method can generate quadrature nodal evaluations at the cost of a polynomial evaluation, which may increase its efficiency relative to other shooting-based variational integrators. A symmetric version of the method is proposed, and numerical experiments are conducted to exhibit the efficacy and efficiency of the method.