The equation can also be written as m d u c d s − − m Γ Γ a b c u a u b = e F c b u b , where Γ Γ a b c is the Christoffel symbol (of the torsion-free metric connection in general relativity), or as m D u c d s = e F c b u b , where D is the covariant differential in general relativity (metric, torsion-free).