Solution
= Solution
For a constant-coefficient differential operator $P(D)=\sum_{|\beta|\leq m}a_\beta D^\beta$, a hypersurface with nonzero normal covector $\nu$ is a <non-characteristic hypersurface> precisely when its <principal symbol> $P_m(\nu)=\sum_{|\beta|=m}a_\beta\nu^\beta$ is nonzero. Multiplying the symbol by the conventional factor $i^m$ does not affect this criterion. For the initial line here the normal is $(0,1)$.