  The gravitational field and the source-free electromagnetic field can be
unified preliminarily by the equations in the Riemannian geometry, both are
contractions of im and ik, respectively. So it will be equivalent to the Yang
gravitational equations. From this we can obtain the Lorentz equations of
motion, the first system and second source-free system of Maxwell field
equations. This unification can be included in the gauge theory, and the
unified gauge group is GL(2,C), which is just the same as the gauge group of
the Riemanian manifold.
