Solution
= Solution
The principal symbol is
$$
p(\tau,\xi)=\tau^2-(1+u^2)|\xi|^2.
$$
Its value on $dt$ is $1$, independently of the data, so the initial plane is non-characteristic everywhere.
= Solution
The principal symbol is
$$
p(\tau,\xi)=\tau^2-(1+u^2)|\xi|^2.
$$
Its value on $dt$ is $1$, independently of the data, so the initial plane is non-characteristic everywhere.