Bourgain–Brezis–Mironescu formula for Riesz potentials

We identify the Bourgain-Brezis-Mironescu pointwise limit of the nonlocal potential operator $(1-α)\, I_α(\mathcal D^αf)$, $0<α<1$, where $I_α$ denotes the Riesz potential and $\mathcal D^α$ a nonlinear fractional differential operator. Specifically, for every $f\in C_c^\infty(\mathbb R^n)$ and every $x\in \mathbb R^n$, we show that \begin{equation*} \lim_{α\to 1^-} (1-α)\, I_α(\mathcal D^αf)(x) = K_n\, I_1(|\nabla f|)(x), \end{equation*} where $K_n$ is the geometric constant appearing in the well-known Bourgain-Brezis-Mironescu formula [BBM02]. By a density argument, we further extend this result to every $f\in W^{1,1}(\mathbb R^n)$, obtaining almost everywhere convergence along subsequences.