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The 2-transitive ovoids. (English) Zbl 0652.51013

Let V be a finite classical polar space of finite dimension, f (resp. Q) its associated polarity (quadratic form), \({\mathcal O}\) an ovoid in V (i.e. a set of singular points such that every maximal totally singular subspace meets \({\mathcal O}\) in just one point). \(\Gamma\) denotes the subgroup of those semilinear mappings which preserve the symplectic or unitary polarity f or the quadratic form Q. Two ovoids are called isomorphic iff there exists some element of \(\Gamma\) mapping one onto the other. \(\Gamma_{\{{\mathcal O}\}}\) and \(\Gamma_{({\mathcal O})}\) denote the setwise and pointwise stabilizer of \({\mathcal O}\) in \(\Gamma\), respectively, Aut(\({\mathcal O}):=\Gamma_{\{{\mathcal O}\}}/\Gamma_{({\mathcal O})}.\)
In his main theorem the author gives a classification (up to isomorphism) of ovoids \({\mathcal O}\), if Aut(\({\mathcal O})\) acts two-transitively on the points of \({\mathcal O}\). This classification is carried out in terms of Aut(\({\mathcal O})\) and the group G of isometries of V. With only one exception he finds unique isomorphism classes. All the ovoids occurring in the classification have already been described elsewhere, cf. J. W. P. Hirschfeld [Finite projective spaces of three dimensions (1985; Zbl 0574.51001); p. 46ff], W. M. Kantor [Can. J. Math. 34, 1195- 1207 (1982; Zbl 0467.51004)], and J. A. Thas [Geom. Dedicata 10, 135-143 (1981; Zbl 0458.51010)]. The proof uses the classification of the finite 2-transitive permutation groups.
Reviewer: H.Szambien

MSC:

51E20 Combinatorial structures in finite projective spaces
51E25 Other finite nonlinear geometries
51F25 Orthogonal and unitary groups in metric geometry
20B25 Finite automorphism groups of algebraic, geometric, or combinatorial structures
05B25 Combinatorial aspects of finite geometries
20D99 Abstract finite groups
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References:

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