We establish an estimate for the fundamental solution of the heat equation on a closed Riemannian manifold
M of dimension at least 3, evolving under the Ricci flow. The estimate depends on some constants arising from a Sobolev imbedding theorem. Considering the case when the scalar curvature is positive throughout the manifold, at any time, we will obtain, as a corollary, a bound similar to the
... [Show full abstract] one known for the fixed metric case. Comment: 8 pages