Local well-posedness of the periodic nonlinear Schrödinger equation with a quadratic nonlinearity ${\bar{u}}^2$ in negative Sobolev spaces

We study low regularity local well-posedness of the nonlinear Schrödinger equation (NLS) with the quadratic nonlinearity ${\bar{u}}^2$, posed on one-dimensional and two-dimensional tori. While the relevant bilinear estimate with respect to the $X^{s,b}$-space is known to fail when the regularity $s$ is below some threshold value, we establish local well-posedness for such low regularity by introducing modifications on the $X^{s,b}$-space.