Hyperbolic geometry or hypercycle, then the triangle has no circumscribed circle. As in spherical and elliptical geometry, in hyperbolic geometry if two triangles are similar, they must be congruent. A special polygon in hyperbolic geometry is the regular apeirogon, a uniform polygon with an infinite number of sides. In Euclidean geometry, the only way to construct such a polygon is to make the side lengths tend to zero and the apeirogon is indistinguishable from a circle, or make the interior angles tend to 180 degrees and the apeirogon approaches a straight line. However, in hyperbolic geometry, a regular apeirogon has sides of
Ovoid (projective geometry) 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. Semi-ovoids are used in the construction of examples of Möbius geometries. Ovoid (projective geometry) In projective geometry an ovoid is a sphere like pointset (surface) in a projective space of dimension . Simple examples in a real projective space are hyperspheres (quadrics). The essential