Sharp unconditional well-posedness of the 2-$d$ periodic cubic hyperbolic nonlinear Schrödinger equation

Abstract: We study semilinear local well-posedness of the two-dimensional periodic cubic hyperbolic nonlinear Schr\"odinger equation (HNLS) in Fourier-Lebesgue spaces. By employing the Fourier restriction norm method, we first establish sharp semilinear local well-posedness of HNLS in Fourier-Lebesgue spaces (modulo the endpoint case), including almost scaling-critical Fourier-Lebesgue spaces. Then, by adapting the normal form approach, developed by the second author with Guo and Kwon (2013) and by the second and third authors (2021), to the current hyperbolic setting, we establish sharp unconditional uniqueness of HNLS within the semilinear local well-posedness regime. As a key ingredient to both results, we establish sharp counting estimates for the hyperbolic Schr\"odinger equation. As a byproduct of our analysis, we also obtain sharp unconditional uniqueness of the (usual) two-dimensional periodic cubic nonlinear Schr\"odinger equation in Fourier--Lebesgue spaces for $p \ge 3$.