### Abstract

We study the linear stability of smooth steady states of the evolution equation h_{t} = -(f(h)h_{xxx})_{x} - (g(h)h_{x})_{x} - ah under both periodic and Neumann boundary conditions. If a ≠ 0 we assume f ≡ 1. In particular we consider positive periodic steady states of thin film equations, where a = 0 and f, g might have degeneracies such as f(0) = 0 as well as singularities like g(0) = + ∞. If a ≦ 0, we prove each periodic steady state is linearly unstable with respect to volume (area) preserving perturbations whose period is an integer multiple of the steady state's period. For area-preserving perturbations having the same period as the steady state, we prove linear instability for all a if the ratio g/f is a convex function. Analogous results hold for Neumann boundary conditions. The rest of the paper concerns the special case of a = 0 and power-law coefficients f(y) = y^{n} and g(y) = By^{m}. We characterize the linear stability of each positive periodic steady state under perturbations of the same period. For steady states that do not have a linearly unstable direction, we find all neutral directions. Surprisingly, our instability results imply a nonexistence result: there is a large range of exponents m and n for which there cannot be two positive periodic steady states with the same period and volume.

Original language | English (US) |
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Pages (from-to) | 3-51 |

Number of pages | 49 |

Journal | Archive for Rational Mechanics and Analysis |

Volume | 154 |

Issue number | 1 |

DOIs | |

State | Published - Jan 1 2000 |

### ASJC Scopus subject areas

- Analysis
- Mathematics (miscellaneous)
- Mechanical Engineering