Sobolev Mappings Between RCD Spaces and Applications to Harmonic Maps: A Heat Kernel Approach

In this paper, we investigate a Sobolev map f from a finite dimensional RCD space (X, d(X), m(X)) to a finite dimensional non-collapsed compact RCD space (Y, dY, HN). It is proved that if the image f (X) is smooth in a weak sense (which is satisfied if the pushforward measure f#mX is absolutely continuous with respect to the Hausdorff measure H-N, or if (Y, dY, H-N) is smooth in a weak sense), then the pull-back f*gY of the Riemannian metric gY of (Y, d(Y), H-N) is well defined as an L1-tensor on X, the minimal weak upper gradient G f off can be written by using f*gY, and it coincides with the local slope Lip f for mX-almost everywhere points in X when f is Lipschitz. In particular, the last statement gives a nonlinear analogue of Cheeger's differentiability theorem for Lipschitz functions on metric measure spaces. Moreover,these results allow us to define the energy of f. It is also proved that the energy coincides with the Korevaar-Schoen energy up to by multiplying a dimensional positive constant. In order to achieve this, we use a smoothing of gY via the heat kernel embedding f(t) : Y ? L-2(Y, H-N), which is established by Ambrosio-Portegies-Tewodrose and the first-named author (Ambrosio et al. in J Funct Anal 280:108968, 2021). Moreover,we improve the regularity of Ot, which plays a key role to get the above results. As an application, we show that (Y, d(Y)) is isometric to the N-dimensional standard unit sphere in RN+1 and f is a minimal isometric immersion if and only if (X, d(X), m(X)) is non-collapsed up to a multiplication of a constant to m(X), and f is an eigenmap whose eigenvalues coincide with the essential dimension of (X, d(X), m(X)), which gives a positive answer to a remaining problem from a previous work [49] by the first-named author. This approach, using the heat kernel embedding instead of using Nash's one, to the study of energies of maps between possibly singular spaces seems new even for closed Riemannian manifolds.