# Serie IX, Nr 4

## Surface of order four. The surface consists of ten parts, which are connected in twelve A1 double points.

Mathematical description
This model belongs to the Kummer's series of surfaces with tetrahedral symmetry. Kummer studied surfaces of degree 4 in complex projective space, given by:

φ2=λpqrs,
where φ=0 is the equation of a sphere (of degree 2):
φ = (x2 + y2 + z2 - μk2)
and p,q,r and s are lineair equations:
```                 p = z - k + x√(2),
q = z - k - x√(2),
r = -z - k + y√(2),
s = -z - k - y√(2).
```
The equation pqrs=0 is a regular tetrahedron concentric with the sphere. The four sides of the tetrahedron have equations p=0,q=0,r=0 and s=0. The parameter μ is the radius of the sphere φ=0. When the sphere meets the edges of the tetrahedron, singularities arise. For μ=1 there are no real intersection points.
The model represents the case then μ=4/3 and λ=1/2. In this case, the sphere cuts each edge of the tetrahedron twice. This is a surface in the four dimensional space, given by the equation:
(x2 + y2 + z2 - 4/3 k2)2 = 1/2 [(z - k)2 - 2x2] [(z + k)2 - 2y2]
The model shows the same surface in the third dimension by taking k=50 mm. It consists of four congruent parts, touching each other in the double points. For general values of &lambda>0, this gives twelve ordinary double points A1.

Title model and translation
Fläche vierter Ordnung. Die Fläche besteht aus zehn Teilen, die in zwölf conischen Knotenpunkten zusammenhängen. Surface of order four. The surface consists of ten parts, which are connected in twelve A1 double points.

Text on Sticker and translation
Fl. 4. Ord. mit 4 dobbelebenen.
Surface of order four with four double planes.

Designer
Dr. Kummer
Berlin in 1883

Company
Martin Schilling in Halle a.S.

Original price
Mark 26,50

Dimension
11-11 cm

Material
Plaster

Literature
Schilling, M., Catalog mathematischer Modelle für den höheren mathematischen Unterricht, Leipzig: Verlag von Martin Schilling, 1911