Near polygon

In mathematics, a near polygon is an incidence geometry introduced by Ernest E. Shult and Arthur Yanushka in 1980. Shult and Yanushka showed the connection between the so-called tetrahedrally closed line-systems in Euclidean spaces and a class of point-line geometries which they called near polygons. These structures generalise the notion of generalized polygon as every generalized 2n-gon is a near 2n-gon of a particular kind. Near polygons were extensively studied and connection between them and dual polar spaces was shown in 1980s and early 1990s. Some sporadic simple groups, for example the Hall-Janko group and the Mathieu groups, act as automorphism groups of near polygons. A near 2d-gon is an incidence structure (), where is the set of points, is the set of lines and is the incidence relation, such that: The maximum distance between two points (the so-called diameter) is d. For every point and every line there exists a unique point on which is nearest to . Note that the distance are measured in the collinearity graph of points, i.e., the graph formed by taking points as vertices and joining a pair of vertices if they are incident with a common line. We can also give an alternate graph theoretic definition, a near 2d-gon is a connected graph of finite diameter d with the property that for every vertex x and every maximal clique M there exists a unique vertex x in M nearest to x. The maximal cliques of such a graph correspond to the lines in the incidence structure definition. A near 0-gon (d = 0) is a single point while a near 2-gon (d = 1) is just a single line, i.e., a complete graph. A near quadrangle (d = 2) is same as a (possibly degenerate) generalized quadrangle. In fact, it can be shown that every generalized 2d-gon is a near 2d-gon that satisfies the following two additional conditions: Every point is incident with at least two lines. For every two points x, y at distance i < d, there exists a unique neighbour of y at distance i − 1 from x.