Lifespan Lower Estimates for a Strongly Damped Semilinear Wave Equation

We consider a strongly damped semilinear wave equation with initial data prescribed as $(\varrho \phi ,\varrho h)$, where the profiles are fixed and only the amplitude $\varrho >0$ is allowed to vary. The question addressed here is how this rescaling affects a guaranteed lower bound for the maximal existence time. We show that the solution exists at least on a time interval of length comparable to ${\varrho }^{-(p-2)}$. The proof is based on the growth of a quadratic phase-space norm: after the source term is estimated by the relevant Sobolev embedding, the problem reduces to a scalar differential inequality. The constants produced in the argument are independent of $\varrho$, so the dependence on the initial amplitude remains explicit throughout.