“…For the basic fractional diffusion equation, given by the special case F = G = 0 and a = b = 0, the solution admits a series representation via separation of variables, which, in combination with the asymptotics of the Mittag-Leffler function, yields bounds on the time derivatives of u in various spatial norms [17,18]. One may also represent the solution in terms of a fractional resolvent [19,20,21]. These simple approaches no longer work in the general case, and the analysis that follows relies instead on the tools used in our study [1] of well-posedness: energy methods and a fractional Gronwall inequality.…”