On sharp global well-posedness and Ill-posedness for a fifth-order KdV-BBM type equation

We consider the Cauchy problem associated to the recently derived higher order hamiltonian model for unidirectional water waves and prove global existence for given data in the Sobolev space $H^s$, $s\ge 1$. We also prove an ill-posedness result by showing that the flow-map is not continuous if the given data has Sobolev regularity $s<1$. The results obtained in this work are sharp.