Unconditional well-posedness for some nonlinear periodic one-dimensional dispersive equations

Abstract: We consider the Cauchy problem for one-dimensional dispersive equations with a general nonlinearity in the periodic setting. Our main hypotheses are both that the dispersive operator behaves for high frequencies as a Fourier multiplier by $ i |\xi|^\alpha \xi $, with $ 1\le \alpha \le 2 $, and that the nonlinear term is of the form $ \partial_x f(u) $ where $ f $ is the sum of an entire series with infinite radius of convergence. Under these conditions, we prove the unconditional local well-posedness of the Cauchy problem in $H^{s}(\mathbb{T})$ for $ s\ge 1-\frac{\alpha}{2(\alpha+1)}$. This leads to some global existence results above the energy space $ H^{\alpha/2}(\mathbb{T}) $, for $ \alpha \in [\sqrt{2},2]$.