A space in which it holds is called Ptolemaic. For instance, consider the four-vertex cycle graph, shown in the figure, with all edge lengths equal to 1. The sum of the products of opposite sides is 2.
A space occupied by something is called Ptolemaic. For instance, consider the four-vertex cycle graph shown in the figure, with all edges having a length of 1. The sum of the products of opposite sides is 2.