WELL-POSEDNESS OF QUADRATIC HARTREE TYPE EQUATIONS BELOW L2

This paper studies the Cauchy problem for the nonlinear Schr¨odinger equation i∂tu − ∂2 xu = f(u) in one space dimension. The nonlinear interaction f(u) is a linear combination of (V ∗x u)u, (V ∗x ¯u)u, (V ∗x u)¯u and (V ∗x ¯u)¯u, where V (x) is a locally integrable function whose Fourier transform satisfies | ˆ V (ξ)| ≲ ⟨ξ⟩−m for some m ≥ 0. The Cauchy problem is well-posed in Hs for s > −(m/2+1/4); furthermore, if f(u) contains only the first and the last types of nonlinear terms, then the Cauchy problem is well-posed for s > −(m/2+3/4). The proof is based on bilinear estimates in Xs,b spaces.