Examples of separating coordinates for the Klein–Gordon equation in 2+1‐dimensional flat space–time

We consider the domains of those pseudo‐orthogonal coordinate systems in flat 2+1‐dimensional space–time which allow for the separation of the Klein–Gordon equation by a product ansatz and which were characterized by Kalnins and Miller in connection with the symmetry group of the wave equation. The horizons of these domains which were constructed as enveloping surfaces of the common tangent null planes of the coordinate surfaces turn out to be ruled surfaces, generated by the totality of tangents of a null curve. This paper is a report on a longer one containing the horizons and domains of the full number of 87 separating coordinate systems.