Ill-posedness issues for a class of parabolic equations

We prove that the Cauchy problem for the one-dimensional parabolic equations , with initial data in Hs(R), cannot be solved by an iterative scheme based on the Duhamel formula for s < -1 if (k, d) = (2, 0) and s < sc(k, d) = [frac12] - (2 - d)/(k - 1) otherwise. This exactly completes the positive results on the Cauchy problem in Hs(R) for these equations and shows the particularity of the case (k, d) = (2, 0), for which we prove that the critical space Hsc(R) = H-3/2(R), by standard scaling arguments, cannot be reached. Our results also hold in the periodic setting.