  We study genus zero wall-crossing for a family of moduli spaces introduced
recently by Fan-Farvis-Ruan. The family has a wall and chamber structure
relative to a positive rational parameter. For a Fermat quasi-homogeneous
polynomial W (not necessarily Calabi-Yau type), we study natural generating
functions of invariants associated to these moduli spaces. Our wall-crossing
formula relates the generating functions by showing that they all lie on the
same Lagrangian cone associated to the Fan-Jarvis-Ruan-Witten theory of W. For
arbitrarily small parameter, a specialization of our generating function is a
hypergeometric series called the big I-function which determines the entire
Lagrangian cone. As a special case of our wall-crossing, we obtain a new
geometric interpretation of the Landau-Ginzburg mirror theorem.
