“…This special issue collects selected works from participants of RMMM 2019 that are related to their presentations. The overall focus is as wide as the needs for mathematics in computational PDEs, addressing a posteriori error control [2,5] and adaptivity [1,3,7,8], reliable methods for space-time problems [3,4,12], non-standard numerical discretizations [2,4,6,9], and iterative solvers and optimal preconditioning [7,8,10,11].…”