The $\alpha$-SQG patch problem is illposed in $C^{2,\beta }$ and $W^{2,p}$

Abstract

We consider the patch problem for the $\alpha$-SQG system with the values $\alpha =0$ and $\alpha =\frac{1}{2}$ being the 2D Euler and the SQG equations respectively. It is well-known that the Euler patches are globally wellposed in non-endpoint $C^{k,\beta }$ Hölder spaces, as well as in $W^{2,p},$ $1<p<\infty$ spaces. In stark contrast to the Euler case, we prove that for $0<\alpha <\frac{1}{2}$, the $\alpha$-SQG patch problem is strongly illposed in \emph{every} $C^{2,\beta }$ Hölder space with $\beta <1$. Moreover, in a suitable range of regularity, the same strong illposedness holds for \emph{every} $W^{2,p}$ Sobolev space unless $p=2$.

arXiv commentv2: minor revision, to appear in CPAM

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