Segre's theorem


In projective geometry, Segre's theorem, named after the Italian mathematician Beniamino Segre, is the statement:
This statement was assumed 1949 by the two Finnish mathematicians G. Järnefelt and P. Kustaanheimo and its proof was published in 1955 by B. Segre.
A finite pappian projective plane can be imagined as the projective closure of the real plane, where the real numbers are replaced by a finite field. Odd order means that is odd. An oval is a curve similar to a circle : any line meets it in at most 2 points and through any point of it there is exactly one tangent. The standard examples are the nondegenerate projective conic sections.
In pappian projective planes of even order greater than four there are ovals which are not conics. In an infinite plane there exist ovals, which are not conics. In the real plane one just glues a half of a circle and a suitable ellipse smoothly.
The proof of Segre's theorem, shown below, uses the 3-point version of Pascal's theorem and a property of a finite field of odd order, namely, that the product of all the nonzero elements equals -1.

Definition of an oval

If the line is an exterior line; in case a tangent line and if the line is a secant line.
For finite planes we have a more convenient characterization:
;Theorem:
Let be an oval in a pappian projective plane of characteristic.

is a nondegenerate conic if and only if statement '
holds:
;Proof:
Let the projective plane be coordinatized inhomogeneously over a field
such that is the tangent at, the x-axis is the tangent at the point and contains the point. Furthermore, we set

The oval can be described by a function such that:
The tangent at point will be described using a function such that its equation is
Hence
I: if is a non degenerate conic we have and and one calculates easily that are collinear.
II: If is an oval with property ', the slope of the line is equal to the slope of the line, that means:
With one gets
and yield
A consequence of and is
Hence is a nondegenerate conic.
Remark:
Property is fulfilled for any oval in a pappian projective plane of characteristic 2 with a nucleus. Hence in this case is also true for non-conic ovals.

Segre's theorem and its proof

;Theorem:
Any oval in a finite pappian projective plane of odd order is a nondegenerate conic section.
;Proof:
For the proof we show that the oval has property ' of the 3-point version of Pascal's theorem.
Let be any triangle on and defined as described in '.
The pappian plane will be coordinatized inhomogeneously over a finite field
, such that and is the common point of the tangents at and. The oval can be described using a bijective function :
For a point, the expression is the slope of the secant Because both the functions and are bijections from
to, and a bijection from onto , where is the slope of the tangent at , for we get


Hence
Because the slopes of line and tangent
both are, it follows that
This is true for any triangle.
So: of the 3-point Pascal theorem holds and the oval is a non degenerate conic.