“…According to the method of computing the reduced-order coefficients, the RB methods can be classified into two categories [2,18]: intrusive and non-intrusive. In the former, the ROM is firstly established by using Galerkin projection [4,19,20,21,22] or other means [23,24], which yields a finite-dimensional dynamical system with the smallest possible DoF [16,25,26], and then the reduced-order coefficients are calculated by solving the ROM with the time scheme (e.g., the second order leap-frog (LF 2 ) scheme). MOR for FDTD [27,28], DGTD [16,26], and hybridizable discontinuous Galerkin (HDG) [4,29] methods based on the projection technique has proved to be a powerful method to save computational cost for the PDEs.…”