Solution
= Solution
The second-order principal symbol of the heat operator evaluated on $dt-dx_1$ is $-1$, up to the overall sign convention. Thus this tilted hypersurface is non-characteristic.
= Solution
The second-order principal symbol of the heat operator evaluated on $dt-dx_1$ is $-1$, up to the overall sign convention. Thus this tilted hypersurface is non-characteristic.