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Arcs and ovals in infinite {$K$}-clan geometry

Arcs and ovals in infinite \(K\)-clan geometry
Authors: Bader, Laura; O'Keefe, Christine M.;

Arcs and ovals in infinite {$K$}-clan geometry

Abstract

Let \(F=GF(q)\) be the Galois field with \(q\) elements. A \(q\)-clan is a set \({\mathcal C}\) of \(q\), \(2\times 2\) matrices over \(F\) whose pairwise differences are anisotropic, i.e., define quadratic forms that represent 0 only trivially. Associated with a \(q\)-clan \({\mathcal C}\) is a flock \({\mathcal F}({\mathcal C})\) of a quadratic cone in \(PG(3,q)\), (and with the flock a line spread of \(PG(3,q)\) and hence translation planes of order \(q^2)\), a generalized quadrangle \(GQ({\mathcal C})\) with parameters \((q^2,q)\). When \(q\) is a power of 2, there are also ovals in \(PG(2,q)\) derived from \({\mathcal C}\), along with subquadrangles of order \(q\). \textit{F. De Clerck} and \textit{H. Van Maldeghem} [Bull. Belg. Math. Soc.-Simon Stevin 3, No. 1, 399-415 (1994; Zbl 0811.51004)] replaced \(F\) with an infinite field \(K\) and studied the problem of replacing \({\mathcal C}\) with a \(K\)-clan so as to obtain an infinite generalized quadrangle. More recently \textit{L. Bader} and the reviewer took a different approach to this problem [J. Geom. 63, No. 1-2, 1-16 (1998)]. Also, N. L. Johnson and his coauthors have studied \(K\)-clans and flocks, with some attention to the associated translation planes. See especially \textit{V. Jha} and \textit{N. L. Johnson} [J. Geom. 57, No. 1-2, 123-150 (1996; Zbl 0866.51002)] and \textit{N. L. Johnson} and the reviewer (in Lect. Notes Pure Appl. Math. 190, 51-122 (1997) and their references. The paper under review, however, seems to be the first to extend to the infinite case a study of the associated ovals in \(PG(2,K)\) when the characteristic of \(K\) is 2, along with their corresponding subquadrangles. The definition of \(K\)-clan was chosen to correspond to the situation where the natural candidate \({\mathcal F}({\mathcal C})\) for a flock really is a flock. Then a \(K\)-clan \({\mathcal C}\) is by definition a 4-gonal \(K\)-clan provided the natural candidate \(GQ({\mathcal C})\) for a generalized quadrangle really is one. De Clerck and Van Maldeghem first showed that this is the case if and only if each ``derivation'' of the flock \({\mathcal F}({\mathcal C})\) is again a flock, and algebraic conditions on the coordinatizing functions that give \({\mathcal C}\) were worked out that are equivalent to having \({\mathcal C}\) be 4-gonal. When generalizing to the infinite case, it always seems necessary to have more complicated definitions and/or special restrictions on the field. One of the main results of the present paper is the following: Let \(K\) be a perfect field of characteristic 2. Let \({\mathcal C}\) be a 4-gonal \(K\)-clan. Then each natural candidate \({\mathcal O}\) for an oval of \(PG(2,K)\) is indeed an oval, and the corresponding generalized quadrangle \(GQ({\mathcal C})\) has a subquadrangle isomorphic to the usual generalized quadrangle \(T_2({\mathcal O})\) constructed by J. Tits from the oval \({\mathcal O}\).

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Italy
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Keywords

flocks, generalized quadrangles, infinite generalised polygons, 51E12, Nonlinear incidence geometry, infinite \(K\)-clan, generalized quadrangle, Arcs; ovals; infinite generalised polygons, Blocking sets, ovals, \(k\)-arcs, ovals, Arcs, Generalized quadrangles and generalized polygons in finite geometry, 51E21

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
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