THE FLUX HOMOMORPHISM AND CENTRAL EXTENSIONS OF DIFFEOMORPHISM GROUPS
概要
Let D be a closed unit disk in dimension two and G_<rel> the group of symplectomorphisms on D preserving the origin and the boundary ∂D pointwise. We consider the flux homomorphism on G_<rel> and construct a central R-extension called the flux extension. We determine the Euler class of this extension and investigate the relation among the extension, the group 2-cocycle defined by Ismagilov, Losik, and Michor, and the Calabi invariant of D.