Four-dimensional shrinkers with nonnegative Ricci curvature

In this paper, we investigate classifications of $4$-dimensional simply connected complete noncompact nonflat shrinkers satisfying $Ric+\mathrm{Hess}\,f=\tfrac 12g$ with nonnegative Ricci curvature. One one hand, we show that if the sectional curvature $K\le 1/4$ or the sum of smallest two eigenvalues of Ricci curvature has a suitable lower bound, then the shrinker is isometric to $\mathbb{R}\times\mathbb{S}^3$. We also show that if the scalar curvature $R\le 3$ and the shrinker is asymptotic to $\mathbb{R}\times\mathbb{S}^3$, then the Euler characteristic $\chi(M)\geq 0$ and equality holds if and only if the shrinker is isometric to $\mathbb{R}\times\mathbb{S}^3$. On the other hand, we prove that if $K\le 1/2$ (or the bi-Ricci curvature is nonnegative) and $R\le\tfrac{3}{2}-\delta$ for some $\delta\in (0,\tfrac{1}{2}]$, then the shrinker is isometric to $\mathbb{R}^2\times\mathbb{S}^2$. The proof of these classifications mainly depends on the asymptotic analysis by the evolution of eigenvalues of Ricci curvature, the Gauss-Bonnet-Chern formula with boundary and the integration by parts.