H\"older regularity of harmonic functions on metric measure spaces

We introduce the H\"older regularity condition for harmonic functions on metric measure spaces and prove that under mild volume regular condition and upper heat kernel estimate, the H\"older regularity condition, the weak Bakry-\'Emery non-negative curvature condition, the heat kernel H\"older continuity with or without exponential terms and the heat kernel near-diagonal lower bound are equivalent. As applications, firstly, we prove the validity of the so-called generalized reverse H\"older inequality on the Sierpi\'nski carpet cable system, which was left open by Devyver, Russ, Yang (Int. Math. Res. Not. IMRN (2023), no. 18, 15537-15583). Secondly, we prove that two-sided heat kernel estimates alone imply gradient estimate for the heat kernel on strongly recurrent fractal-like cable systems, which improves the main results of the aforementioned paper. Thirdly, we obtain H\"older (Lipschitz) estimate for heat kernel on general metric measure spaces, which extends the classical Li-Yau gradient estimate for heat kernel on Riemannian manifolds.