On curves and surfaces in illumination geometry.
We give an example of a set of points in such that, for any partition of into triples, there exists a line stabbing of the triangles determined by the triples.
We show that a central linear mapping of a projectively embedded Euclidean -space onto a projectively embedded Euclidean -space is decomposable into a central projection followed by a similarity if, and only if, the least singular value of a certain matrix has multiplicity . This matrix is arising, by a simple manipulation, from a matrix describing the given mapping in terms of homogeneous Cartesian coordinates.