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Discrete Routh reduction

Sameer M Jalnapurkar1, Melvin Leok2, Jerrold E Marsden3 and Matthew West4

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This paper develops the theory of Abelian Routh reduction for discrete mechanical systems and applies it to the variational integration of mechanical systems with Abelian symmetry. The reduction of variational Runge–Kutta discretizations is considered, as well as the extent to which symmetry reduction and discretization commute. These reduced methods allow the direct simulation of dynamical features such as relative equilibria and relative periodic orbits that can be obscured or difficult to identify in the unreduced dynamics. The methods are demonstrated for the dynamics of an Earth orbiting satellite with a non-spherical J2 correction, as well as the double spherical pendulum. The J2 problem is interesting because in the unreduced picture, geometric phases inherent in the model and those due to numerical discretization can be hard to distinguish, but this issue does not appear in the reduced algorithm, where one can directly observe interesting dynamical structures in the reduced phase space (the cotangent bundle of shape space), in which the geometric phases have been removed. The main feature of the double spherical pendulum example is that it has a non-trivial magnetic term in its reduced symplectic form. Our method is still efficient as it can directly handle the essential non-canonical nature of the symplectic structure. In contrast, a traditional symplectic method for canonical systems could require repeated coordinate changes if one is evoking Darboux' theorem to transform the symplectic structure into canonical form, thereby incurring additional computational cost. Our method allows one to design reduced symplectic integrators in a natural way, despite the non-canonical nature of the symplectic structure.


PACS

02.30.Xx Calculus of variations

02.60.Cb Numerical simulation; solution of equations

03.65.Vf Phases: geometric; dynamic or topological

02.60.Gf Algorithms for functional approximation

MSC

53D20 Momentum maps; symplectic reduction

65Kxx Mathematical programming, optimization and variational techniques

65C20 Models, numerical methods (See also 68U20)

Subjects

Mathematical physics

Computational physics

Quantum information and quantum mechanics

Dates

Issue 19 (12 May 2006)

Received 24 August 2005, in final form 5 January 2006

Published 24 April 2006



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