On completeness of the Moutard transformations
  In this paper we solve positively the problem of (local) density of the "potentials" M(x,y) of the Moutard equation, u_{xy} = M(x,y) u, u=u(x,y), (used in many papers for construction of exact solutions of (2+1)-dimensional integrable systems) obtainable from a given initial potential with consecutive Moutard transformations. 
E. I. Ganzha (Krasnoyarsk State Pedagogical University)