The geometry seminar scheduled for Tuesday 24th November **has been cancelled**. The speaker would have been François Charette (Max Planck, Bonn). We hope to invite him to come another time. For interest, we keep his title and abstract here: *Quantum Reidemeister torsion and open Gromov-Witten invariants*.

Lagrangian Floer homology is a tool that gives information about Lagrangian submanifolds in symplectic topology. For example, it has been used to proved one of Arnold’s conjectures about existence (and lower bounds) of closed periodic orbits of the Hamiltonian flow. However, this homology often vanishes. In this talk, I will show how torsion of an acyclic chain complex can be used to extract information out of the Floer chain complex in this case. This will be interpreted as certain rational functions of open Gromov Witten invariants.