RIGIDITY OF GRADIENT ALMOST RICCI SOLITONS

摘要

In this paper, we show that either, a Euclidean space R~n, or a standard sphere S~n, is the unique manifold with nonnega-tive scalar curvature which carries a structure of a gradient almost Ricci soliton, provided this gradient is a non trivial conformal vector field. Moreover, in the spherical case the field is given by the first eigenfunction of the Laplacian. Finally, we shall show that a compact locally conformally flat almost Ricci soliton is isometric to Euclidean sphere S~n provided an integral condition holds.