Solution
= Solution
The <d'Alembert operator>[wave operator] has principal symbol
$$
p(\tau,\xi)=-\tau^2+|\xi|^2.
$$
The conormal to the initial plane is $dt$, and $p(dt)=-1\ne0$. The plane is therefore non-characteristic.
= Solution
The <d'Alembert operator>[wave operator] has principal symbol
$$
p(\tau,\xi)=-\tau^2+|\xi|^2.
$$
The conormal to the initial plane is $dt$, and $p(dt)=-1\ne0$. The plane is therefore non-characteristic.