Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 105 / 4 / b / Solution

Write $Lu=L_{0}u+(a^{ij}-A^{ij})D_{ij}u$, with repeated indices summed. The triangle inequality and the Cauchy-Schwarz inequality over the $n^2$ coefficient pairs giveChooseThen the strict coefficient bound in the question implies