The Prescribed $Q$-Curvature Flow for Arbitrary Even Dimension in a Critical Case

In this paper, we study the prescribed $Q$-curvature flow equation on a arbitrary even dimensional closed Riemannian manifold $(M,g)$, which was introduced by S. Brendle in cite{B2003}, where he proved the flow exists for long time and converges at infinity if the GJMS operator is weakly positive with trivial kernel and $int_M Qdmu < (n-1)!Volleft( S^n right) $. In this paper we study the critical case that $int_M Qdmu = (n-1)!Volleft( S^n right)$, we will prove the convergence of the flow under some geometric hypothesis. In particular, this gives a new proof of Li-Li-Lius existence result in cite{LLL2012} in dimensiona 4 and extend the work of Li-Zhu cite{LZ2019} in dimension 2 to general even dimensions. In the proof, we give a explicit expression of the limit of the corresponding energy functional when the blow up occurs.