Well-Posedness for KdV-Type Equations with Second-Order Derivative Nonlinearities

We study a class of complex-valued KdV-type equations with cubic second-order derivative nonlinearities and arbitrary complex coefficients. For sufficiently small initial data in $L^2$, we prove local well-posedness in $H^s(\mathbb{R})$ for $s\ge 3\mathrm{/}4$. The key ingredient is a family of dyadic resolution spaces $Z_k=X_k+Y_k$, designed to overcome the logarithmic divergence arising from high$\ast$low$\ast$low interactions in which both derivatives fall on the highest-frequency factor. For the completely integrable third-order Kaup–Newell flow, we establish global-in-time $H^s(\mathbb{R})$ bounds for $0\le s<1$ and global well-posedness in $H^s(\mathbb{R})$ for every $s\ge 3\mathrm{/}4$, with no smallness assumption on the initial data.