U(1)-invariant special Lagrangian 3-folds. II. Existence of singular solutions

AbstractThis is the second of three papers studying special Lagrangian 3-submanifolds (SLV 3-folds) N in C3 invariant under the U(1)-action eiθ:(z1,z2,z3)↦(eiθz1,e−iθz2,z3), using analytic methods. If N is such a 3-fold then |z1|2−|z2|2=2a on N for some a∈R. Locally, N can be written as a kind of graph of functions u,v:R2→R satisfying a nonlinear Cauchy–Riemann equation depending on a, so that u+iv is like a holomorphic function of x+iy.The first paper studied the case when a is nonzero. Then u,v are smooth and N is nonsingular. It proved existence and uniqueness for solutions of two Dirichlet problems derived from the equations on u,v. This implied existence and uniqueness for a large class of nonsingular U(1)-invariant SL 3-folds in C3, with boundary conditions.In this paper and its sequel we focus on the case a=0. Then the nonlinear Cauchy–Riemann equation is not always elliptic. Because of this there may be points (x,0) where u,v are not differentiable, corresponding to singular points of N. This paper is concerned largely with technical analytic issues, and the sequel with the geometry of the singularities of N. We prove a priori estimates for derivatives of solutions of the nonlinear Cauchy–Riemann equation, and use them to show existence and uniqueness of weak solutions u,v to the two Dirichlet problems when a=0, which are continuous and weakly differentiable. This gives existence and uniqueness for a large class of singular U(1)-invariant SL 3-folds in C3, with boundary conditions