“…Then, traditional state estimation and/or control methods from finite-dimensional system theory can be exploited for the resulting early lumped systems. The widely-utilized early lumping methods are reported as: modal analysis (Galerkin's method) [47,48], finite difference [49], proper orthogonal decomposition (POD) [50,51], finite element method (FEM) [52], FEM Galerkin cubic b-splines method [53], etc. In contrast, late lumping means that one conducts observer, filter, and/or controller design steps on the original infinite-dimensional space and applies spatial approximation later for simulation purposes, where the backstepping method [54,55], the adjoint-based approach [56], and the Crank-Nicolson method [57] are widely used.…”