Conservation and Breakdown of Modulus of Continuity in the Propagation of Loglog Vortex for 2D Euler equation

In this paper, we investigate the persistency of loglog singular structure for 2D Euler equation under the perturbation of continuous function. First, we prove that when loglog vortex is perturbed by the function with modulus of continuity $\mu (r)={(\log \frac{1}{r})}^{-\alpha }(0<\alpha <1)$, the loglog vortex is propagated while the perturbation term maintaining same modulus of continuity. We then show that this result is sharp: There exists a smooth function such that when the loglog vortex is perturbed by the smooth function, the norm of perturbation term corresponding to modulus of continuity $\mu (r)={(\log \frac{1}{r})}^{-\alpha }$ instantly blows up for all $\alpha >1$.