Sharp Lifespan Estimates for a Semilinear Wave Equation with Nonlinear Damping

Abstract

We study the maximal existence time $T^{\ast }(\varrho )$ of the solution of the semilinear wave equation $u_{tt}-\Delta u=u|u{|}^{p-1}-u_t|u_t{|}^{q-1}$ in a bounded domain, with Dirichlet boundary condition and initial data $(\varrho f,\varrho g)$, where $1<q<p$ and the amplitude $\varrho$ is large. For nontrivial $f$ and sufficiently large $\varrho$, the concavity method gives $T^{\ast }(\varrho )\le C{\varrho }^{-(p-1)/2}$ for $q\le 2p/(p+1)$ and $T^{\ast }(\varrho )\le C{\varrho }^{-(p-q)/q}$ for $q>2p/(p+1)$, whereas the energy method gives a lower bound of order ${\varrho }^{1-p}$ only. We prove lower bounds with the same exponents as the upper ones, so that $T^{\ast }(\varrho )\asymp {\varrho }^{-\vartheta (p,q)}$ with $\vartheta (p,q)=\min \{(p-1)/2,(p-q)/q\}$. The proof rests on a hyperbolic rescaling which converts the large amplitude into a dilation of the domain and a coefficient ${\varrho }^{(q(p+1)-2p)/2}$ in front of the damping term, on a local existence theory in uniformly local energy norms whose existence time does not depend on that coefficient, and on a quantitative use of the dissipation when the coefficient is large. The threshold $2p/(p+1)$ is the value of $q$ at which the damping term is invariant under the rescaling. No attempt is made to optimize the constants: sharpness is meant throughout at the level of the exponent.

26 pages