“…To determine the reduced-order or expansion coefficients of the POD basis functions during the online stage, the MOR methods are generally grouped into two families: intrusive MOR (IMOR) and non-intrusive MOR (NIMOR) methods. In IMOR method, the POD method combined with projection techniques are usually used to reduce the complexity of classical numerical methods, such as the finite difference (FD) [32,54,51], FE [21,43,18], FV [31,26,41], hybridizable discontinuous Galerkin (HDG) [44], and DGTD [24,23] methods, where the Galerkin procedure is the most popular choice for the projection [46]. Besides, in order to avoid the unrewarding repeated computations in the IMOR methods, some reduced-order extrapolated schemes have successively been established by Luo's research team since 2013 [28,29,33,34,36,35,30,55].…”