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[[File:GQ(2,2), the Doily.svg|thumb|GQ(2,2), the Doily]]

In [[geometry]], a '''generalized quadrangle''' is an [[incidence structure]] whose main feature is the lack of any triangles (yet containing many quadrangles). A generalized quadrangle is by definition a [[polar space]] of rank two. They are the {{nowrap|[[generalized n-gon]]s}} with ''n'' = 4. They are also precisely the [[Partial geometry|partial geometries]] pg(''s'',''t'',α) with α = 1.

==Definition==

A generalized quadrangle is an incidence structure (''P'',''B'',I), with I ⊆ ''P'' × ''B'' an [[incidence relation]], satisfying certain [[axiom]]s. Elements of ''P'' are by definition the ''points'' of the generalized quadrangle, elements of ''B'' the ''lines''. The axioms are the following:
* There is an ''s'' (''s'' ≥ 1) such that on every line there are exactly ''s'' + 1 points. There is at most one point on two distinct lines.
* There is a ''t'' (''t'' ≥ 1) such that through every point there are exactly ''t'' + 1 lines. There is at most one line through two distinct points.
* For every point ''p'' not on a line ''L'', there is a unique line ''M'' and a unique point ''q'', such that ''p'' is on ''M'', and ''q'' on ''M'' and ''L''.

(''s'',''t'') are the ''parameters'' of the generalized quadrangle. The parameters are allowed to be infinite. If either ''s'' or ''t'' is one, the generalized quadrangle is called ''trivial''. A generalized quadrangle with parameters (''s'',''t'') is often denoted by GQ(''s'',''t'').

The smallest non-trivial generalized quadrangle is GQ(2,2), whose representation has been dubbed "the doily" by Stan Payne in 1973.

==Properties==

* <math>|P|=(s t+1)(s+1)</math>
* <math>|B|=(s t+1)(t+1)</math>
* <math>(s+t)|st(s+1)(t+1)</math>
* <math>s\neq 1 \Longrightarrow t\leq s^2</math>
* <math>t\neq 1 \Longrightarrow s\leq t^2</math>

==Graphs==
[[File:GQ24.svg|250px|right|thumb| [[Line graph]] of '''generalized quadrangle''' {{nowrap|GQ(2,4)}}]]

There are two interesting graphs that can be obtained from a generalized quadrangle.
* The ''collinearity graph'' having as vertices the points of a generalized quadrangle, with the collinear points connected. This graph is a [[strongly regular graph]].
* The ''incidence graph'' whose vertices are the points and lines of the generalized quadrangle and two vertices are adjacent if one is a point, the other a line and the point lies on the line. The incidence graph of a generalized quadrangle is characterized by being a [[Connected graph|connected]], [[bipartite graph]] with [[diameter (graph theory)|diameter]] four and [[girth (graph theory)|girth]] eight. Incidence graphs of configurations are today generally called [[Levi graph]]s, but the original Levi graph was the incidence graph of the GQ(2,2).

==Duality==

If (''P'',''B'',I) is a generalized quadrangle with parameters (''s'',''t''), then (''B'',''P'',I<sup>-1</sup>), with I<sup>-1</sup> the inverse incidence relation, is also a generalized quadrangle. This is the ''dual generalized quadrangle''. Its parameters are (''t'',''s''). Even if ''s'' = ''t'', the dual structure need not be isomorphic with the original structure.

==Classical generalized quadrangles==
When looking at the different cases for [[polar space]]s of rank at least three, and extrapolating them to rank 2, one finds these (finite) generalized quadrangles :

* A hyperbolic [[quadric]] <math>Q^+(3,q)</math>, a parabolic quadric <math>Q(4,q)</math> and an elliptic quadric <math>Q^-(5,q)</math> are the only possible quadrics in projective spaces over finite fields with projective index 1. We find these parameters respectively :
: <math>Q(3,q) :\ s=q,t=1</math> (this is just a grid)
: <math>Q(4,q) :\ s=q,t=q</math>
: <math>Q(5,q) :\ s=q,t=q^2</math>
* A hermitian variety <math>H(n,q^2)</math> has projective index 1 if and only if n is 3 or 4. We find :
: <math> H(3,q^2) :\ s=q^2,t=q</math>
: <math>H(4,q^2) :\ s=q^2,t=q^3</math>
* A symplectic polarity in <math>PG(2d+1,q)</math> has a maximal isotropic subspace of dimension 1 if and only if <math>d=1</math>. Here, we find a generalized quadrangle <math>W(3,q)</math>, with <math>s=q,t=q</math>.

The generalized quadrangle derived from <math>Q(4,q)</math> is always isomorphic with the dual of <math>W(3,q)</math>, and they are both self-dual and thus isomorphic to each other if and only if <math>q</math> is even.

==Non-classical examples==

* Let ''O'' be a [[hyperoval]] in <math>PG(2,q)</math> with ''q'' an even [[prime power]], and embed that projective (desarguesian) plane <math>\pi</math> into <math>PG(3,q)</math>. Now consider the incidence structure <math>T_2^{*}(O)</math> where the points are all points not in <math>\pi</math>, the lines are those not on <math>\pi</math>, intersecting <math>\pi</math> in a point of ''O'', and the incidence is the natural one. This is a ''(q-1,q+1)''-generalized quadrangle.
* Let ''q'' be a [[prime power]] (odd or even) and consider a symplectic polarity <math>\theta</math> in <math>PG(3,q)</math>. Choose a random point ''p'' and define <math>\pi=p^{\theta}</math>. Let the lines of our incidence structure be all absolute lines not on <math>\pi</math> together with all lines through ''p'' which are not on <math>\pi</math>, and let the points be all points of <math>PG(3,q)</math> except those in <math>\pi</math>. The incidence is again the natural one. We obtain once again a ''(q-1,q+1)''-generalized quadrangle

==Restrictions on parameters==

By using grids and dual grids, any [[integer]] ''z'', ''z'' ≥ 1 allows generalized quadrangles with parameters (1,''z'') and (''z'',1). Apart from that, only the following parameters have been found possible until now, with ''q'' an arbitrary [[prime power]] :

: <math> (q,q)</math>
: <math> (q,q^2)</math> and <math> (q^2,q)</math>
: <math> (q^2,q^3)</math> and <math> (q^3,q^2)</math>
: <math> (q-1,q+1)</math> and <math> (q+1,q-1)</math>

==References==
* [[S. E. Payne]] and [[J. A. Thas]]. Finite generalized quadrangles. Research Notes in Mathematics, 110. Pitman (Advanced Publishing Program), Boston, MA, 1984. vi+312 pp. ISBN 0-273-08655-3
* [[Koen Thas]]. Symmetry in finite generalized quadrangles. Frontiers in Mathematics. Birkhäuser Verlag, Basel, 2004. xxii+214 pp. ISBN 3-7643-6158-1

[[Category:Incidence geometry]]
[[Category:Set families]]

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en>AnomieBOT: Dating maintenance tags: {{Unreferenced}}

140.113.156.133 at 03:44, 16 September 2011