In mathematics, specifically projective geometry, a configuration in the plane consists of a finite set of points, and a finite arrangement of lines, such that each point is incident to the same number of lines and each line is incident to the same number of points.
Although certain specific configurations had been studied earlier (for instance by Thomas Kirkman in 1849), the formal study of configurations was first introduced by Theodor Reye in 1876, in the second edition of his book Geometrie der Lage, in the context of a discussion of Desargues' theorem. Ernst Steinitz wrote his dissertation on the subject in 1894, and they were popularized by Hilbert and Cohn-Vossen's 1932 book Anschauliche Geometrie, reprinted in English as .
Configurations may be studied either as concrete sets of points and lines in a specific geometry, such as the Euclidean or projective planes (these are said to be realizable in that geometry), or as a type of abstract incidence geometry. In the latter case they are closely related to regular hypergraphs and biregular bipartite graphs, but with some additional restrictions: every two points of the incidence structure can be associated with at most one line, and every two lines can be associated with at most one point. That is, the girth of the corresponding bipartite graph (the Levi graph of the configuration) must be at least six.
Notation
A configuration in the plane is denoted by (), where is the number of points, the number of lines, the number of lines per point, and the number of points per line. These numbers necessarily satisfy the equation
as this product is the number of point-line incidences (flags).
Configurations having the same symbol, say (), need not be isomorphic as incidence structures. For instance, there exist three different (93 93) configurations: the Pappus configuration and two less notable configurations.
In some configurations, and consequently, . These are called symmetric or balanced configurations and the notation is often condensed to avoid repetition. For example, (93 93) abbreviates to (93).
Examples
Notable projective configurations include the following: (11), the simplest possible configuration, consisting of a point incident to a line. Often excluded as being trivial. (32), the triangle. Each of its three sides meets two of its three vertices, and vice versa. More generally any polygon of sides forms a configuration of type () (43 62) and (62 43), the complete quadrangle and complete quadrilateral respectively. (73), the Fano plane. This configuration exists as an abstract incidence geometry, but cannot be constructed in the Euclidean plane. (83), the Möbius–Kantor configuration. This configuration describes two quadrilaterals that are simultaneously inscribed and circumscribed in each other. It cannot be constructed in Euclidean plane geometry but the equations defining it have nontrivial solutions in complex numbers. (93), the Pappus configuration. (94 123), the Hesse configuration of nine inflection points of a cubic curve in the complex projective plane and the twelve lines determined by pairs of these points. This configuration shares with the Fano plane the property that it contains every line through its points; configurations with this property are known as Sylvester–Gallai configurations due to the Sylvester–Gallai theorem that shows that they cannot be given real-number coordinates. (103), the Desargues configuration. (124 163), the Reye configuration. (125 302), the Schläfli double six, formed by 12 of the 27 lines on a cubic surface (153), the Cremona–Richmond configuration, formed by the 15 lines complementary to a double six and their 15 tangent planes (166), the Kummer configuration. (214), the Grünbaum–Rigby configuration. (273), the Gray configuration (354), Danzer's configuration. (6015), the Klein configuration.
Duality of configurations
The projective dual of a configuration () is a () configuration in which the roles of "point" and "line" are exchanged. Types of configurations therefore come in dual pairs, except when taking the dual results in an isomorphic configuration. These exceptions are called self-dual configurations and in such cases .
The number of () configurations
The number of nonisomorphic configurations of type (), starting at , is given by the sequence
1, 1, 3, 10, 31, 229, 2036, 21399, 245342, ...