’s. All these tensors served to represent various possible types of structure, geometry or curvature.
In a previous chapter we mentioned these tensors. First, there was the Riemann-Christoffel tensor
. It was a tensor of the fourth order, as is indicated by the four indices underlying the letter
. Then there was a second-order symmetrical tensor
. Lastly, there was an invariant of curvature
, no longer a tensor in the ordinary sense of the word. By differentiating and combining these magnitudes, we could of course obtain others, but the ones we have mentioned are the simplest.