A blow-up result for the wave equation with localized initial data: the
scale-invariant damping and mass term with combined nonlinearities
Abstract
We are interested in this article in studying the damped wave equation
with localized initial data, in the
\textit{scale-invariant case} with mass term and two
combined nonlinearities. More precisely, we consider the following
equation: \begin{displaymath} \d (E)
\hspace{1cm} u_{tt}-\Delta
u+\frac{\mu}{1+t}u_t+\frac{\nu^2}{(1+t)^2}u=|u_t|^p+|u|^q,
\quad \mbox{in}\
\R^N\times[0,\infty),
\end{displaymath} with small initial data. Under some
assumptions on the mass and damping coefficients, $\nu$
and $\mu>0$, respectively, we show that
blow-up region and the lifespan bound of the solution of $(E)$ remain
the same as the ones obtained in \cite{Our2} in the
case of a mass-free wave equation, {\it i.e.} $(E)$
with $\nu=0$. Furthermore, using in part the
computations done for $(E)$, we enhance the result in
\cite{Palmieri} on the Glassey conjecture for the
solution of $(E)$ with omitting the nonlinear term
$|u|^q$. Indeed, the blow-up region is extended
from $p \in (1, p_G(N+\sigma)]$,
where $\sigma$ is given by
\eqref{sigma} below, to $p \in (1,
p_G(N+\mu)]$ yielding, hence, a better estimate of
the lifespan when
$(\mu-1)^2-4\nu^2<1$.
Otherwise, the two results coincide. Finally, we may conclude that the
mass term {\it has no influence} on the dynamics of
$(E)$ (resp. $(E)$ without the nonlinear term
$|u|^q$), and the conjecture we made in
\cite{Our2} on the threshold between the blow-up and
the global existence regions obtained holds true here.