The Meaning of Relativity / Four lectures delivered at Princeton University, May, 1921

by Albert Einstein

, then the difference of these two,

is also a vector. Since this is the case for an arbitrary choice of the

, it follows that

is a tensor, which we designate as the co-variant derivative of the tensor of the first rank (vector). Contracting this tensor, we obtain the divergence of the contra-variant tensor

. In this we must observe that according to (70),