On self-similar solutions of a multi-phase Stefan problem in the half-line

We study self-similar solutions of a multi-phase Stefan problem for a heat equation on the half-line x>0 with a constant initial data and with Dirichlet or Neumann boundary conditions. In the case of Dirichlet boundary condition we prove that a nonlinear algebraic system for determination of the free boundaries is gradient one and the corresponding potential is an explicitly written strictly convex and coercive function. Therefore, there exists a unique minimum point of the potential, coordinates of this point determine free boundaries and provide the desired solution. In the case of Neumann boundary condition we demonstrate that the problem may have solutions with different numbers (called types) of phase transitions. For each fixed type n the system for determination of the free boundaries is again gradient and the corresponding potential is proved to be strictly convex and coercive, but in some wider non-physical domain. In particular, a solution of type n is unique and can exist only if the minimum point of the potential belongs to the physical domain. We provide an explicit criteria for existence of solutions of any type n. Due to the rather complicated structure of the set of solutions, neither existence nor uniqueness of a solution to Stefan-Neumann problem are guaranteed. Bibliography: 5 titles.