A formal series approach to averaging: Exponentially small error estimates
Philippe Chartier Ander Murua Jesús María Sanz-Serna
The techniques, based on formal series and combinatorics, used nowadays to analyze numerical integrators may be applied to perform high-order averaging in oscillatory periodic or quasi-periodic dynamical systems. When this approach is employed, the averaged system may be written in terms of (i) scalar coefficients that are universal, i.e. independent of the system under consideration and (ii) basis functions that may be written in an explicit, systematic way in terms of the derivatives of the Fourier coefficients of the vector field being averaged. The coefficients may be recursively computed in a simple fashion. We show that this approach may be used to obtain exponentially small error estimates, as those first derived by Neishtadt. All the constants that feature in the estimates have a simple explicit expression.
keywords: Averaging exponentially small error estimates B-series high-order averaging quasi-stroboscopic averaging Lie algebras Lie groups. Magnus expansions Chen-Fliess series
Solving highly-oscillatory NLS with SAM: Numerical efficiency and long-time behavior
Philippe Chartier Norbert J. Mauser Florian Méhats Yong Zhang
In this paper, we present the Stroboscopic Averaging Method (SAM), recently introduced in [7,8,10,13], which aims at numerically solving highly-oscillatory differential equations. More specifically, we first apply SAM to the Schrödinger equation on the $1$-dimensional torus and on the real line with harmonic potential, with the aim of assessing its efficiency: as compared to the well-established standard splitting schemes, the stiffer the problem is, the larger the speed-up grows (up to a factor $100$ in our tests). The geometric properties of SAM are also explored: on very long time intervals, symmetric implementations of the method show a very good preservation of the mass invariant and of the energy. In a second series of experiments on $2$-dimensional equations, we demonstrate the ability of SAM to capture qualitatively the long-time evolution of the solution (without spurring high oscillations).
keywords: invariants Hamiltonian PDEs nonlinear Schrödinger. stroboscopic averaging Highly-oscillatory evolution equation

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