Four-dimensional Gradient Ricci Solitons Gradient shrinking Ricci Solitons and Modified Sectional Curvature

Abstract: We investigate four-dimensional gradient shrinking Ricci solitons with positive modified sectional curvature. Our first main result shows that if the norm of the self-dual Weyl tensor and the scalar curvature satisfy a certain sharp pinching condition --closely related, in a precise sense, to that of K\"ahler metric --then the soliton is necessarily locally K\"ahler. We further obtain a characterization theorem and a weighted integral gap result for compact gradient shrinking Ricci solitons with bounded modified sectional curvature. In addition, we establish a Hitchin-Thorpe type inequality for compact four-dimensional Ricci solitons, providing new topological constraints on such manifolds.