, vanish at the origin, the calculation of the
for this manifold, by (88), is very simple at the origin. We have
Since the relation
is universally co-variant, and since all points of the manifold are geometrically equivalent, this relation holds for every system of co-ordinates, and everywhere in the manifold. In order to avoid confusion with the four-dimensional continuum, we shall, in the following, designate quantities that refer to the three-dimensional continuum by Greek letters, and put
We now proceed to apply the field equations (96) to our special case. From (119) we get for the four-dimensional manifold,
