Non-singular solutions to the normalized Ricci flow equation
  In this paper we study non-singular solutions of Ricci flow on a closed manifold of dimension at least 4. Amongst others we prove that, if M is a closed 4-manifold on which the normalized Ricci flow exists for all time t>0 with uniformly bounded sectional curvature, then the Euler characteristic $\chi (M)\ge 0$. Moreover, the 4-manifold satisfies one of the following \noindent (i) M is a shrinking Ricci solition;   \noindent (ii) M admits a positive rank F-structure;   \noindent (iii) the Hitchin-Thorpe type inequality holds 2\chi (M)\ge 3|\tau(M)| where $\chi (M)$ (resp. $\tau(M)$) is the Euler characteristic (resp. signature) of M. 
Fuquan Fang, Yuguang Zhang, Zhenlei Zhang