no attempt to give a comprehensive review here. We only mention two recent relevant papers by Himonas et al [@Han_Himonas_2011], [@Himonas_2011] where the interested reader can find further references on analytic and Gevrey regularity properties for KdV-type equations. In fact, we are interested in a much stronger effect of formation of meromorphic solution out of nonsmooth data. More specifically, in the current paper, we are concerned with the following problem.

\[pb1\] Given the Cauchy problem for the KdV equation[^2] $$\begin{cases}
\partial _{t}u-6u\partial _{z}u+\partial _{z}^{3}u=0 \\ 
u|_{t=0}=q\end{cases},  \label{eq1.1}$$describe the largest possible class of (non-smooth) initial data $q$ which evolve into functions $u(z,t)$ meromorphic with respect to $z$ for any $t>0$.

Meromorphic (or, more generally, analytic) solutions have of course been intensively studied since the boom around integrable systems started in the late 60s. A pure soliton (reflectionless) solution, historically the first explicit solution, is meromorphic on the whole complex plane having infinitely many double poles. This fact is of course a trivial observation immediately following from the explicit formula for multisoliton solutions. We emphasize that how those poles interact is not obvious at all. This question was raised back in earlier 70s by Kruskal and has been followed up by many. 