# Ovoid (projective geometry)

To the definition of an ovoid: t tangent, s secant line

In projective geometry an ovoid is a sphere like pointset (surface) in a projective space of dimension d ≥ 3. Simple examples in a real projective space are hyperspheres (quadrics). The essential geometric properties of an ovoid ${\displaystyle {\mathcal {O}}}$ are:

1. Any line intersects ${\displaystyle {\mathcal {O}}}$ in at most 2 points,
2. The tangents at a point cover a hyperplane (and nothing more), and
3. ${\displaystyle {\mathcal {O}}}$ contains no lines.

Property 2) excludes degenerated cases (cones,...). Property 3) excludes ruled surfaces (hyperboloids of one sheet, ...).

An ovoid is the spatial analog of an oval in a projective plane.

An ovoid is a special type of a quadratic set.

Ovoids play an essential role in constructing examples of Möbius planes and higher dimensional Möbius geometries.

## Definition of an ovoid

• In a projective space of dimension d ≥ 3 a set ${\displaystyle {\mathcal {O}}}$  of points is called an ovoid, if
(1) Any line g meets ${\displaystyle {\mathcal {O}}}$  in at most 2 points.

In the case of ${\displaystyle |g\cap {\mathcal {O}}|=0}$ , the line is called a passing (or exterior) line, if ${\displaystyle |g\cap {\mathcal {O}}|=1}$  the line is a tangent line, and if ${\displaystyle |g\cap {\mathcal {O}}|=2}$  the line is a secant line.

(2) At any point ${\displaystyle P\in {\mathcal {O}}}$  the tangent lines through P cover a hyperplane, the tangent hyperplane, (i.e., a projective subspace of dimension d − 1).
(3) ${\displaystyle {\mathcal {O}}}$  contains no lines.

From the viewpoint of the hyperplane sections, an ovoid is a rather homogeneous object, because

• For an ovoid ${\displaystyle {\mathcal {O}}}$  and a hyperplane ${\displaystyle \varepsilon }$ , which contains at least two points of ${\displaystyle {\mathcal {O}}}$ , the subset ${\displaystyle \varepsilon \cap {\mathcal {O}}}$  is an ovoid (or an oval, if d = 3) within the hyperplane ${\displaystyle \varepsilon }$ .

For finite projective spaces of dimension d ≥ 3 (i.e., the point set is finite, the space is pappian[1]), the following result is true:

• If ${\displaystyle {\mathcal {O}}}$  is an ovoid in a finite projective space of dimension d ≥ 3, then d = 3.
(In the finite case, ovoids exist only in 3-dimensional spaces.)[2]
• In a finite projective space of order n >2 (i.e. any line contains exactly n + 1 points) and dimension d = 3 any pointset ${\displaystyle {\mathcal {O}}}$  is an ovoid if and only if ${\displaystyle |{\mathcal {O}}|=n^{2}+1}$  and no three points are collinear (on a common line).[3]

Replacing the word projective in the definition of an ovoid by affine, gives the definition of an affine ovoid.

If for an (projective) ovoid there is a suitable hyperplane ${\displaystyle \varepsilon }$  not intersecting it, one can call this hyperplane the hyperplane ${\displaystyle \varepsilon _{\infty }}$  at infinity and the ovoid becomes an affine ovoid in the affine space corresponding to ${\displaystyle \varepsilon _{\infty }}$ . Also, any affine ovoid can be considered a projective ovoid in the projective closure (adding a hyperplane at infinity) of the affine space.

## Examples

### In real projective space (inhomogeneous representation)

1. ${\displaystyle {\mathcal {O}}=\{(x_{1},...,x_{d})\in {\mathbb {R} }^{d}\;|\;x_{1}^{2}+\cdots +x_{d}^{2}=1\}\ ,}$  (hypersphere)
2. ${\displaystyle {\mathcal {O}}=\{(x_{1},...,x_{d})\in {\mathbb {R} }^{d}\;|x_{d}=x_{1}^{2}+\cdots +x_{d-1}^{2}\;\}\;\cup \;\{{\text{point at infinity of }}x_{d}{\text{-axis}}\}}$

These two examples are quadrics and are projectively equivalent.

Simple examples, which are not quadrics can be obtained by the following constructions:

(a) Glue one half of a hypersphere to a suitable hyperellipsoid in a smooth way.
(b) In the first two examples replace the expression x12 by x14.

Remark: The real examples can not be converted into the complex case (projective space over ${\displaystyle {\mathbb {C} }}$ ). In a complex projective space of dimension d ≥ 3 there are no ovoidal quadrics, because in that case any non degenerated quadric contains lines.

But the following method guarantees many non quadric ovoids:

• For any non-finite projective space the existence of ovoids can be proven using transfinite induction.[4][5]

### Finite examples

• Any ovoid ${\displaystyle {\mathcal {O}}}$  in a finite projective space of dimension d = 3 over a field K of characteristic ≠ 2 is a quadric.[6]

The last result can not be extended to even characteristic, because of the following non-quadric examples:

• For ${\displaystyle K=GF(2^{m}),\;m}$  odd and ${\displaystyle \sigma }$  the automorphism ${\displaystyle x\mapsto x^{(2^{\frac {m+1}{2}})}\;,}$

the pointset

${\displaystyle {\mathcal {O}}=\{(x,y,z)\in K^{3}\;|\;z=xy+x^{2}x^{\sigma }+y^{\sigma }\}\;\cup \;\{{\text{point of infinity of the }}z{\text{-axis}}\}}$  is an ovoid in the 3-dimensional projective space over K (represented in inhomogeneous coordinates).
Only when m = 1 is the ovoid ${\displaystyle {\mathcal {O}}}$  a quadric.[7]
${\displaystyle {\mathcal {O}}}$  is called the Tits-Suzuki-ovoid.

## Criteria for an ovoid to be a quadric

An ovoidal quadric has many symmetries. In particular:

• Let be ${\displaystyle {\mathcal {O}}}$  an ovoid in a projective space ${\displaystyle {\mathfrak {P}}}$  of dimension d ≥ 3 and ${\displaystyle \varepsilon }$  a hyperplane. If the ovoid is symmetric to any point ${\displaystyle P\in \varepsilon \setminus {\mathcal {O}}}$  (i.e. there is an involutory perspectivity with center ${\displaystyle P}$  which leaves ${\displaystyle {\mathcal {O}}}$  invariant), then ${\displaystyle {\mathfrak {P}}}$  is pappian and ${\displaystyle {\mathcal {O}}}$  a quadric.[8]
• An ovoid ${\displaystyle {\mathcal {O}}}$  in a projective space ${\displaystyle {\mathfrak {P}}}$  is a quadric, if the group of projectivities, which leave ${\displaystyle {\mathcal {O}}}$  invariant operates 3-transitively on ${\displaystyle {\mathcal {O}}}$ , i.e. for two triples ${\displaystyle A_{1},A_{2},A_{3},\;B_{1},B_{2},B_{3}}$  there exists a projectivity ${\displaystyle \pi }$  with ${\displaystyle \pi (A_{i})=B_{i},\;i=1,2,3}$ .[9]

In the finite case one gets from Segre's theorem:

• Let be ${\displaystyle {\mathcal {O}}}$  an ovoid in a finite 3-dimensional desarguesian projective space ${\displaystyle {\mathfrak {P}}}$  of odd order, then ${\displaystyle {\mathfrak {P}}}$  is pappian and ${\displaystyle {\mathcal {O}}}$  is a quadric.

## Generalization: semi ovoid

Removing condition (1) from the definition of an ovoid results in the definition of a semi-ovoid:

A point set ${\displaystyle {\mathcal {O}}}$  of a projective space is called a semi-ovoid if

the following conditions hold:

(SO1) For any point ${\displaystyle P\in {\mathcal {O}}}$  the tangents through point ${\displaystyle P}$  exactly cover a hyperplane.
(SO2) ${\displaystyle {\mathcal {O}}}$  contains no lines.

A semi ovoid is a special semi-quadratic set[10] which is a generalization of a quadratic set. The essential difference between a semi-quadratic set and a quadratic set is the fact, that there can be lines which have 3 points in common with the set and the lines are not contained in the set.

Examples of semi-ovoids are the sets of isotropic points of an hermitian form. They are called hermitian quadrics.

As for ovoids in literature there are criteria, which make a semi-ovoid to a hermitian quadric. See, for example[11].

Semi-ovoids are used in the construction of examples of Möbius geometries.

## Notes

1. ^ Dembowski 1968, p. 28
2. ^ Dembowski 1968, p. 48
3. ^ Dembowski 1968, p. 48
4. ^ W. Heise: Bericht über ${\displaystyle \kappa }$ -affine Geometrien, Journ. of Geometry 1 (1971), S. 197–224, Satz 3.4.
5. ^ F. Buekenhout: A Characterization of Semi Quadrics, Atti dei Convegni Lincei 17 (1976), S. 393-421, chapter 3.5
6. ^ Dembowski 1968, p. 49
7. ^ Dembowski 1968, p. 52
8. ^ H. Mäurer: Ovoide mit Symmetrien an den Punkten einer Hyperebene, Abh. Math. Sem. Hamburg 45 (1976), S.237-244
9. ^ J. Tits: Ovoides à Translations, Rend. Mat. 21 (1962), S. 37–59.
10. ^ F. Buekenhout: A Characterization of Semi Quadrics, Atti dei Convegni Lincei 17 (1976), S. 393-421.
11. ^ K.J. Dienst: Kennzeichnung hermitescher Quadriken durch Spiegelungen, Beiträge zur geometrischen Algebra (1977), Birkhäuser-Verlag, S. 83-85.

## References

• Dembowski, Peter (1968), Finite geometries, Ergebnisse der Mathematik und ihrer Grenzgebiete, Band 44, Berlin, New York: Springer-Verlag, ISBN 3-540-61786-8, MR 0233275