Kähler-Ricci flow with unbounded curvature
Let $g(t)$ be a complete solution to the Ricci flow on a noncompact manifold such that $g(0)$ is Kahler. We prove that if $|Rm(g(t))|_{g(t)}\le a/t$ for some $a>0$, then $g(t)$ is Kahler for $t>0$. We prove that there is a constant $ a(n)>0$ depending only on $n$ such that the following is true: Suppose $g(t)$ is a complete solution to the Kahler-Ricci flow on a noncompact $n$-dimensional complex manifold such that $g(0)$ has nonnegative holomorphic bisectional curvature and such that $|Rm(g(t))|_{g(t)}\le a(n)/t$, then $g(t)$ has nonnegative holomorphic bisectional curvature for $t>0$. These generalize the results by W. S. Shi. As corollaries, we prove that (i) any complete noncompact Kahler manifold with nonnegative complex sectional curvature with maximum volume growth is biholomorphic to $ C^n$; and (ii) there is $ε(n)>0$ depending only on $n$ such that if $(M^n,g_0)$ is a complete noncompact Kahler manifold of complex dimension $n$ with nonnegative holomorphic bisectional curvature and maximum volume growth and if $(1+ε(n))^{-1}h\le g_0\le (1+ε(n))h$ for some Riemannian metric $h$ with bounded curvature, then $M$ is biholomorphic to $C^n$.