Solution
= Solution
For $\phi=t-x_1$, the conormal is $dt-dx_1$, and
$$
p(d\phi)=-1+1=0.
$$
This null hyperplane is a <characteristic hypersurface> of the wave equation.
= Solution
For $\phi=t-x_1$, the conormal is $dt-dx_1$, and
$$
p(d\phi)=-1+1=0.
$$
This null hyperplane is a <characteristic hypersurface> of the wave equation.