# Hyperboloid

A non-closed central surface of the second order. One distinguishes between two types of hyperboloids: the one-sheet and the two-sheet hyperboloid.

Figure: h048360a

Figure: h048360b

In a suitable coordinate system (see Fig. a, Fig. b) the equation of a one-sheet hyperboloid is

while that of a two-sheet hyperboloid is

The numbers , and (and segments of such lengths) are known as the semi-axes of the hyperboloid. Sections of a hyperboloid by planes passing through the -axis are hyperbolas. Sections of a hyperboloid by planes perpendicular to the -axis are ellipses. The section of a one-sheet hyperboloid by the plane is said to be a gorge ellipse. A hyperboloid has three planes of symmetry. The cone defined by the equation

is called the asymptotic cone. If , the hyperboloid is said to be regular. A hyperboloid with two equal semi-axes is said to be a hyperboloid of rotation. A one-sheet hyperboloid is a ruled surface; the equations of the rectilinear generators passing through a given point have the form

#### Comments

#### References

[a1] | M. Berger, "Geometry" , 1–2 , Springer (1987) (Translated from French) |

[a2] | D. Hilbert, S.E. Cohn-Vossen, "Geometry and the imagination" , Chelsea (1952) (Translated from German) |

**How to Cite This Entry:**

Hyperboloid.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Hyperboloid&oldid=14677