Abstract
We construct a new Evans function for quasi-periodic solutions to the linearisation of the sine-Gordon equation about a periodic travelling wave. This Evans function is written in terms of fundamental solutions to a Hill’s equation. Applying the Evans-Krein function theory of Kollár and Miller (SIAM Rev 56(1):73–123, 2014) to our Evans function, we provide a new method for computing the Krein signatures of simple characteristic values of the linearised sine-Gordon equation. By varying the Floquet exponent parametrising the quasi-periodic solutions, we compute the linearised spectra of periodic travelling wave solutions of the sine-Gordon equation and track dynamical Hamiltonian–Hopf bifurcations via the Krein signature. Finally, we show that our new Evans function can be readily applied to the general case of the nonlinear Klein–Gordon equation with a non-periodic potential.
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Acknowledgements
The authors would like to thank Peter Miller as well as the referees for their helpful suggestions for improving our paper, Dave Smith for his comments which clarified our notation, and the referees for their helpful suggestions for improving our paper. W. Clarke would like to thank Henrik Schumacher for his insightful discussion about speeding up our code. R. Marangell acknowledges the support of the Australian Research Council under Grant DP200102130.
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Clarke, W.A., Marangell, R. A New Evans Function for Quasi-Periodic Solutions of the Linearised Sine-Gordon Equation. J Nonlinear Sci 30, 3421–3442 (2020). https://doi.org/10.1007/s00332-020-09655-4
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DOI: https://doi.org/10.1007/s00332-020-09655-4
Keywords
- Evans function
- Periodic travelling waves
- Spectral stability
- Non-linear Klein–Gordon equation
- Krein signature
- Hamiltonian–Hopf bifurcations