Abstract. We prove a Davies type double integral estimate for the heat kernel H(y, t; x, l) under the Ricci flow. As a result, we give an affirmative answer to a question proposed in [8]. Moreover, we apply the Davies type estimate to provide a new proof of the Gaussian upper and lower bounds of H(y, t; x, l) which were first shown in [6].
IntroductionOn a complete Riemannian manifold (M n , g ij ), the heat kernel H(x, y, t), is the smallest positive fundamental solution to the heat equation ∂u ∂t = ∆u.( is the distance between U 1 and U 2 .In this paper, we consider the heat kernel of the time-evolving heat equation under the Ricci flow on a complete manifold M n , i.e., where ∆ t is the Laplacian with respect to a complete solution g ij (t), t ∈ [0, T ) and T < ∞, of the following Ricci flow equationThe existence and uniqueness of the heat kernel H(y, t; x, l) to (1.3) were proved in Our main goal is to give an affirmative answer to the question above. More specifically, we prove Theorem 1.2. Let (M n , g ij (t)) be a complete solution to (1.4) on [0, T ) and T < ∞. Suppose that H(y, t; x, l) is the heat kernel of (1.3), and Rc ≥ −K 1 on [0, T ) for some nonnegative constant K 1 . Then for any open sets, U 1 and U 2 , with compact closure in M, and 0 ≤ l < t < T , we havewhere C 0 is a constant depends only on n.Using Theorem 1.2, we are able to provide a new proof of the Gaussian upper bound and lower bounds of H(y, t; x, l) which was first shown by . For the Gaussian upper bound, we have where constants C 1 , C 2 ,, C 3 and C 4 depend only on n.Next, following a method of Li- Tam H(y, t; x, l) ≥ C 5 e −C 6 e C 7 Λ+C 8 K 1 T exp − 4d 2 t (x, y) (t − l)Vol t (B t (x, t−l 8 ))