are the equations of geodetic line; since along the geodetic line considered we have ds ≠ 0, we can choose the parameter λ, as the length of the arc measured along the geodetic line. Then w = 1, and we would get in place of (20c)
Or by merely changing the notation suitably,
"20d"
where we have put, following Christoffel,
"21"
Multiply finally (20d) with gστ (outer multiplication with reference to τ, and inner with respect to σ) we get at last the final form of the equation of the geodetic line—
