“…Indeed, spectral gap conditions have been widely used in the literature to establish the existence of inertial manifolds for many dissipative evolution equations (cf. [1,13,17,18,19,32,34]). However, for a system that lacks the spectral gap condition, in their pioneering work [26], Mallet-Paret and Sell have introduced the so-called spatial averaging method to prove the existence of inertial manifolds for a three-dimensional reaction-diffusion equation.…”