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Klein quartic
In hyperbolic geometry, the Klein quartic, named after Felix Klein, is a compact Riemann surface of genus with the highest possible order automorphism group for this genus, namely order orientation-preserving automorphisms, and automorphisms if orientation may be reversed. As such, the Klein quartic is the Hurwitz surface of lowest possible genus; see Hurwitz's automorphisms theorem. Its (orientation-preserving) automorphism group is isomorphic to , the second-smallest non-abelian simple group. The quartic was first described in .
Klein's quartic occurs in many branches of mathematics, in contexts including representation theory, homology theory, octonion multiplication, Fermat's last theorem, and the Stark-Heegner theorem on imaginary quadratic number fields of class number one; see for a survey of properties.
Originally, the "Klein quartic" referred specifically to the subset of the complex projective plane defined by an algebraic equation. This has a specific Riemannian metric (that makes it a minimal surface in ), under which its Gaussian curvature is not constant. But more commonly (as in this article) it is now thought of as any Riemann surface that is conformally equivalent to this algebraic curve, and especially the one that is a quotient of the hyperbolic plane by a certain cocompact group that acts freely on by isometries. This gives the Klein quartic a Riemannian metric of constant curvature that it inherits from . This set of conformally equivalent Riemannian surfaces is precisely the same as all compact Riemannian surfaces of genus 3 whose conformal automorphism group is isomorphic to the unique simple group of order 168. This group is also known as , and also as the isomorphic group . By covering space theory, the group mentioned above is isomorphic to the fundamental group of the compact surface of genus .
== Closed and open forms ==
It is important to distinguish two different forms of the quartic. The ''closed'' quartic is what is generally meant in geometry; topologically it has genus 3 and is a compact space. The ''open'' or "punctured" quartic is of interest in number theory; topologically it is a genus 3 surface with 24 punctures, and geometrically these punctures are cusps. The open quartic may be obtained (topologically) from the closed quartic by puncturing at the 24 centers of the tiling by regular heptagons, as discussed below. The open and closed quartics have different metrics, though they are both hyperbolic and complete – geometrically, the cusps are "points at infinity", not holes, hence the open quartic is still complete.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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