Past exam of the mathematics course of the University of Cambridge / 2022 / iii / Paper 327 / 3 / i

For the wave operator ${\partial }_x^2-{\partial }_y^2$, use the symboland complexify $\eta$. Its roots $\eta =\pm \xi$ are real for every real $\xi$, so the single horizontal stepnever meets a root and is an explicit Hörmander staircase.