“…z s z t is a symmetric positive semidefinite matrix, with s,t = 1, 2 and (z 1 , z 2 ) ∈ [L 2 (Ω)] 2 , and ∆ K is the patch of elements associated with K. According to [33,34], E ∇ coincides with the difference between the discrete gradient of the density, ∇ρ h , and a corresponding suitable reconstruction, P(∇ρ h ). In the literature, several examples for the recovery operator P are available (see, e.g., [16,19,25,33,34]), which consists of a projection or an average of the discrete gradient across a suitable patch of elements surrounding K. We adopt the area-weighed average over the patch ∆ K ,…”