Abstract In this paper, we consider conformal characterizations of standard sphere in terms of conformal vector fields on closed Riemannian manifolds. We firstly prove that each closed Riemannian manifold with Ricci curvature being non-negative in certain direction and constant scalar curvature is isometric to standard sphere if and only if it admits a non-trivial closed conformal vector field. In the case of non-constant scalar curvature, we show that each closed Riemannian manifold of dimension two with positive Gauss curvature carrying a non-trivial closed conformal vector field is conformal to a round sphere and we generalize the result to high dimensions in two directions.
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