\b[Characteristic.] The second-order <principal symbol> of $i\partial_t+\partial_x^2$ is $\nu_x^2$, which vanishes on the normal $(0,1)$. This uses total differential order, rather than an anisotropic convention that assigns different weights to time and space derivatives.

Solution

Past exam of the mathematics course of the University of Cambridge / 2016 / iii / Paper 105 / 1 / ii / Solution

 Solution (source code)