This project will investigate wall-crossing formulas for a wide class of invariants which appear in a priori different situations in mathematics and physics. Mathematically, those invariants are typically described as virtual Euler characteristics of some moduli spaces. The wall-crossing phenomenon is related to the presence of real codimension one "walls" in the space of parameters, where the invariants jump. In the case of Donaldson-Thomas invariants, the walls live in the moduli space of Bridgeland stability conditions on the ppropriate Calabi-Yau categories. Similar walls also occur in the theory of representations of quivers and cluster algebras. In mirror symmetry, walls correspond to jumps in the number of pseudo-holomorphic discs bounded by the torus fibers of an SYZ Lagrangian fibration. In supersymmetric gauge theories in physics, the number of BPS states jumps across "walls of marginal stability". The Kontsevich-Soibelman wall-crossing formulas for Donaldson-Thomas invariants thus occur in the physics literature on topics such as moduli spaces of vector ultiplets of 4-dimensional supersymmetric theories and supersymmetric black holes. Since these various wall-crossing formulas look so similar, one can ask for a common formalism. The aim of the FRG is to study the underlying "wall-crossing structures" and demonstrate hat the above-mentioned similarities are not coincidental, but rather reflect a deep underlying theory.
It is a frequently encountered situation in mathematics and physics that numerical quantities which in principle depend on various parameters actually are constant for general parameter values (they are "invariants"), but jump along certain "walls" in the parameter space. Wall-crossing formulas describe these "jumps" quantitatively. The subject of wall-crossing has recently become a very active one due to its relevance to a number of different areas of mathematics and physics. The aim of this project is to develop the concept of "wall-crossing structure" rigorously and apply it to problems both old and new in which wall-crossing formulas appear. The results arising from this project will be in demand by both the mathematics and physics communities. The FRG will also build a research community around this coordinated effort, involving a mix of junior and senior researchers, training opportunities for graduate students, and the rganization of several workshops.