On the Spectral Problem Lu=λu′\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{L} u=\lambda u'}$$\end{document} and Applications

We develop a general instability index theory for an eigenvalue problem of the type Lu=λu′\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{L} u=\lambda u'}$$\end{document}, for a class of self-adjoint operators L\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{L}}$$\end{document} on the line R1. More precisely, we construct an Evans-like function to show (a real eigenvalue) instability in terms of a Vakhitov–Kolokolov type condition on the wave. If this condition fails, we show by means of Lyapunov–Schmidt reduction arguments and the Kapitula–Kevrekidis–Sandstede index theory that spectral stability holds. Thus, we have a complete spectral picture, under fairly general assumptions on L\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{L}}$$\end{document}. We apply the theory to a wide variety of examples. For the generalized Bullough–Dodd–Tzitzeica type models, we give instability results for travelling waves. For the generalized short pulse/Ostrovsky/Vakhnenko model, we construct (almost) explicit peakon solutions, which are found to be unstable, for all values of the parameters.