The Cauchy problems for the 2D compressible Euler equations and ideal MHD system are ill-posed in $H^\frac{7}{4}(\mathbb{R}^2)$

Abstract: In a fractional Sobolev space $H^\frac{7}{4}(\mathbb{R}^2)$, we prove the desired low-regularity ill-posedness results for the 2D compressible Euler equations and the 2D ideal compressible MHD system. For the Euler equations, it is sharp with respect to the regularity of the fluid velocity and density. The mechanism behind the obtained ill-posedness is the instantaneous shock formation.