Title: An overdetermined boundary value problem for de Saint-Venant's shaft

Abstract:
Serrin showed that if the shear stress is equal on the outer boundary
of an elastic shaft under torsion, then the shaft is circular. I'll
describe something about the de Saint-Venant theory of torsion and
present Weinberger's solution in the simplest case. The argument uses
the maximum principle and an integral identity. I'll try to make the
talk accessible to beginning graduate students.