# Steiner conic

The Steiner conic or more precisely Steiner's generation of a conic, named after the Swiss mathematician Jakob Steiner, is an alternative method to define a non-degenerate projective conic section in a projective plane over a field.

The usual definition of a conic uses a quadratic form (see Quadric (projective geometry)). Another alternative definition of a conic uses a hyperbolic polarity. It is due to K. G. C. von Staudt and sometimes called a von Staudt conic. The disadvantage of von Staudt's definition is that it only works when the underlying field has odd characteristic (i.e., ${\displaystyle Char\neq 2}$).

## Definition of a Steiner conic

• Given two pencils ${\displaystyle B(U),B(V)}$ of lines at two points ${\displaystyle U,V}$ (all lines containing ${\displaystyle U}$ and ${\displaystyle V}$ resp.) and a projective but not perspective mapping ${\displaystyle \pi }$ of ${\displaystyle B(U)}$ onto ${\displaystyle B(V)}$. Then the intersection points of corresponding lines form a non-degenerate projective conic section[1][2][3] [4] (figure 1)

A perspective mapping ${\displaystyle \pi }$ of a pencil ${\displaystyle B(U)}$ onto a pencil ${\displaystyle B(V)}$ is a bijection (1-1 correspondence) such that corresponding lines intersect on a fixed line ${\displaystyle a}$, which is called the axis of the perspectivity ${\displaystyle \pi }$ (figure 2).

A projective mapping is a finite product of perspective mappings.

Examples of commonly used fields are the real numbers ${\displaystyle \mathbb {R} }$, the rational numbers ${\displaystyle \mathbb {Q} }$ or the complex numbers ${\displaystyle \mathbb {C} }$. The construction also works over finite fields, providing examples in finite projective planes.

Remark: The fundamental theorem for projective planes states,[5] that a projective mapping in a projective plane over a field (pappian plane) is uniquely determined by prescribing the images of three lines. That means that, for the Steiner generation of a conic section, besides two points ${\displaystyle U,V}$ only the images of 3 lines have to be given. These 5 items (2 points, 3 lines) uniquely determine the conic section.

Remark: The notation "perspective" is due to the dual statement: The projection of the points on a line ${\displaystyle a}$ from a center ${\displaystyle Z}$ onto a line ${\displaystyle b}$ is called a perspectivity (see below).[5]

## Example

For the following example the images of the lines ${\displaystyle a,u,w}$ (see picture) are given: ${\displaystyle \pi (a)=b,\pi (u)=w,\pi (w)=v}$. The projective mapping ${\displaystyle \pi }$ is the product of the following perspective mappings ${\displaystyle \pi _{b},\pi _{a}}$: 1) ${\displaystyle \pi _{b}}$ is the perspective mapping of the pencil at point ${\displaystyle U}$ onto the pencil at point ${\displaystyle O}$ with axis ${\displaystyle b}$. 2) ${\displaystyle \pi _{a}}$ is the perspective mapping of the pencil at point ${\displaystyle O}$ onto the pencil at point ${\displaystyle V}$ with axis ${\displaystyle a}$. First one should check that ${\displaystyle \pi =\pi _{a}\pi _{b}}$ has the properties: ${\displaystyle \pi (a)=b,\pi (u)=w,\pi (w)=v}$. Hence for any line ${\displaystyle g}$ the image ${\displaystyle \pi (g)=\pi _{a}\pi _{b}(g)}$ can be constructed and therefore the images of an arbitrary set of points. The lines ${\displaystyle u}$ and ${\displaystyle v}$ contain only the conic points ${\displaystyle U}$ and ${\displaystyle V}$ resp.. Hence ${\displaystyle u}$ and ${\displaystyle v}$ are tangent lines of the generated conic section.

A proof that this method generates a conic section follows from switching to the affine restriction with line ${\displaystyle w}$ as the line at infinity, point ${\displaystyle O}$ as the origin of a coordinate system with points ${\displaystyle U,V}$ as points at infinity of the x- and y-axis resp. and point ${\displaystyle E=(1,1)}$. The affine part of the generated curve appears to be the hyperbola ${\displaystyle y=1/x}$.[2]

Remark:

1. The Steiner generation of a conic section provides simple methods for the construction of ellipses, parabolas and hyperbolas which are commonly called the parallelogram methods.
2. The figure that appears while constructing a point (figure 3) is the 4-point-degeneration of Pascal's theorem.[6]

## Steiner generation of a dual conic

### Definitions and the dual generation

Dualizing (see duality (projective geometry)) a projective plane means exchanging the points with the lines and the operations intersection and connecting. The dual structure of a projective plane is also a projective plane. The dual plane of a pappian plane is pappian and can also be coordinatized by homogenous coordinates. A nondegenerate dual conic section is analogously defined by a quadratic form.

A dual conic can be generated by Steiner's dual method:

• Given the point sets of two lines ${\displaystyle u,v}$ and a projective but not perspective mapping ${\displaystyle \pi }$ of ${\displaystyle u}$ onto ${\displaystyle v}$. Then the lines connecting corresponding points form a dual non-degenerate projective conic section.

A perspective mapping ${\displaystyle \pi }$ of the point set of a line ${\displaystyle u}$ onto the point set of a line ${\displaystyle v}$ is a bijection (1-1 correspondence) such that the connecting lines of corresponding points intersect at a fixed point ${\displaystyle Z}$, which is called the centre of the perspectivity ${\displaystyle \pi }$ (see figure).

A projective mapping is a finite sequence of perspective mappings.

It is usual, when dealing with dual and common conic sections, to call the common conic section a point conic and the dual conic a line conic.

In the case that the underlying field has ${\displaystyle Char=2}$ all the tangents of a point conic intersect in a point, called the knot (or nucleus) of the conic. Thus, the dual of a non-degenerate point conic is a subset of points of a dual line and not an oval curve (in the dual plane). So, only in the case that ${\displaystyle Char\neq 2}$ is the dual of a non-degenerate point conic a non-degenerate line conic.

### Example

For the following example the images of the points ${\displaystyle A,U,W}$ are given: ${\displaystyle \pi (A)=B,\,\pi (U)=W,\,\pi (W)=V}$. The projective mapping ${\displaystyle \pi }$ can be represented by the product of the following perspectivities ${\displaystyle \pi _{B},\pi _{A}}$:

1) ${\displaystyle \pi _{B}}$ is the perspectivity of the point set of line ${\displaystyle u}$ onto the point set of line ${\displaystyle o}$ with centre ${\displaystyle B}$.
2) ${\displaystyle \pi _{A}}$ is the perspectivity of the point set of line ${\displaystyle o}$ onto the point set of line ${\displaystyle v}$ with centre ${\displaystyle A}$.

One easily checks that the projective mapping ${\displaystyle \pi =\pi _{A}\pi _{B}}$ fulfills ${\displaystyle \pi (A)=B,\,\pi (U)=W,\,\pi (W)=V}$. Hence for any arbitrary point ${\displaystyle G}$ the image ${\displaystyle \pi (G)=\pi _{A}\pi _{B}(G)}$ can be constructed and line ${\displaystyle {\overline {G\pi (G)}}}$ is an element of a non degenerate dual conic section. Because the points ${\displaystyle U}$ and ${\displaystyle V}$ are contained in the lines ${\displaystyle u}$, ${\displaystyle v}$ resp.,the points ${\displaystyle U}$ and ${\displaystyle V}$ are points of the conic and the lines ${\displaystyle u,v}$ are tangents at ${\displaystyle U,V}$.

## Notes

1. Coxeter 1993, p. 80
2. Hartmann, p. 38
3. Merserve 1983, p. 65
4. Jacob Steiner’s Vorlesungen über synthetische Geometrie, B. G. Teubner, Leipzig 1867 (from Google Books: (German) Part II follows Part I) Part II, pg. 96
5. Hartmann, p. 19
6. Hartmann, p. 32

## References

• Coxeter, H. S. M. (1993), The Real Projective Plane, Springer Science & Business Media
• Hartmann, Erich, Planar Circle Geometries, an Introduction to Moebius-, Laguerre- and Minkowski Planes (PDF), retrieved 20 September 2014 (PDF; 891 kB).
• Merserve, Bruce E. (1983) [1959], Fundamental Concepts of Geometry, Dover, ISBN 0-486-63415-9