“…For solving the non-homogeneous backward heat equation (i.e. f = 0), several different nontrivial extensions of such quasi-boundary methods were discussed in [11,35,39,44,49,[55][56][57], which involved more sophisticated approximations based on the truncated series . Such quasi-boundary methods were also generalized for solving backward time-dependent and nonlinear parabolic equations, backward time-fractional diffusion problems [30,59,62], and backward time-fractional inverse source problem [61].…”