Hurwitz's automorphisms theorem

In mathematics, Hurwitz's automorphisms theorem bounds the group of automorphisms, via orientation-preserving conformal mappings, of a compact Riemann surface of genus "g" > 1, telling us that the order of the group of such automorphisms is bounded by

:84("g" − 1).

A group for which the maximum is achieved is called a Hurwitz group, and the corresponding Riemann surface a Hurwitz surface. Because compact Riemann surfaces are synonymous with non-singular complex projective algebraic curves, [Technically speaking, there is an equivalence of categories between the category of compact Riemann surfaces with the orientation-preserving conformal maps and the category of non-singular complex projective algebraic curves with the algebraic morphisms.] a Hurwitz surface can also be called a Hurwitz curve. The theorem is due to Adolf Hurwitz, who proved it in 1893.

Interpretation in terms of hyperbolicity

One of the fundamental themes in differential geometry is a trichotomy between the Riemannian manifolds of positive, zero, and negative curvature "K". It manifests itself in many diverse situations and on several levels. In the context of compact Riemann surfaces "X", via the Riemann uniformization theorem, this can be seen as a distinction between the surfaces of different topologies:
* "X" a sphere, a compact Riemann surface of genus zero with "K" > 0;
* "X" a flat torus, or an elliptic curve, a Riemann surface of genus one with "K" = 0;
* and "X" a hyperbolic surface, which has genus greater than one and "K" < 0.

While in the first two cases the surface "X" admits infinitely many conformal automorphisms (in fact, the conformal automorphism group is a connected complex Lie group of dimension three for a sphere and of dimension one for a torus), a hyperbolic Riemann surface only admits a discrete set of automorphisms. Hurwitz's theorem claims that in fact more is true: it provides a uniform bound on the order of the automorphism group as a function of the genus and characterizes those Riemann surfaces for which the bound is sharp.

The idea of a proof and construction of the Hurwitz surfaces

By the uniformization theorem, any hyperbolic surface "X" is covered by the hyperbolic plane. The conformal mappings of the surface correspond to orientation-preserving automorphisms of the hyperbolic plane. By the Gauss-Bonnet theorem, the area of the surface is

: A("X") = 2π &chi;("X") = 4π("g" − 1).

In order to make the automorphism group "G" of "X" as large as possible, we want the area of its fundamental domain "D" for this action to be as small as possible. If the fundamental domain is a triangle with the vertex angles π/p, π/q and π/r, defining a tiling of the hyperbolic plane, then "p", "q", and "r" are integers greater than one, and the area is : A("D") = π(1 − 1/"p" − 1/"q" − 1/"r").

Thus we are asking for integers which make the expression

:1 − 1/"p" − 1/"q" − 1/"r"

strictly positive and as small as possible. A remarkable fact is that this minimal value is 1/42, and

:1 − 1/2 − 1/3 − 1/7 = 1/42

gives a unique (up to permutation) triple of such integers. This would indicate that the order |"G"| of the automorphism group is bounded by

: A("X")/A("D") &le; 168("g" − 1).

However, a more delicate reasoning shows that this is an overestimate by the factor of two, because the group "G" can contain orientation-reversing transformations. For the orientation-preserving conformal automorphisms the bound is 84("g" − 1).

To obtain an example of a Hurwitz group, let us start with a (2,3,7)-tiling of the hyperbolic plane. Its full symmetry group is a triangle group generated by the reflections across the sides of a single fundamental triangle with the angles π/2, π/3 and π/7. Since a reflection flips the triangle and changes the orientation, we can join the triangles in pairs and obtain an orientation-preserving tiling polygon.A Hurwitz surface is obtained by 'closing up' a part of this infinite tiling of the hyperbolic plane to a compact Riemann surface of genus "g". This will necessarily involve exactly 84("g" − 1) double triangle tiles.

From the arguments above it can be inferred that a Hurwitz group "G" is characterized by the property that it is a finite quotient of the group with two generators "a" and "b" and three relations

:$a^2\; =\; b^3\; =\; (ab)^7\; =\; 1,,$

thus "G" is a finite group generated by two elements of orders two and three, whose product is of order seven. More precisely, any Hurwitz surface, that is, a hyperbolic surface that realizes the maximum order of the automorphism group for the surfaces of a given genus, can be obtained by the construction given. This is the last part of the theorem of Hurwitz.

Examples of Hurwitz's groups and surfaces

The smallest Hurwitz group is the special linear group L₂(7), of order 168, and the corresponding curve is the Klein quartic curve.

Next is the Macbeath curve, with automorphism group L₂(8) of order 504. Many more finite simple groups are Hurwitz groups; for instance all but 64 of the alternating groups are Hurwitz groups, the largest non-Hurwitz example being of degree 167. The smallest alternating group that is a Hurwitz group is A₁₅.

Most special linear groups of large rank are Hurwitz groups, harv|Lucchini|Tamburini|Wilson|2000. For lower ranks, fewer such groups are Hurwitz. For "n"_"p" the order of "p" modulo 7, one has that PSL(2,"q") is Hurwitz if and only if either "q"=7 or "q" = "p"^"n"_"p". Indeed, PSL(3,"q") is Hurwitz if and only if "q" = 2, PSL(4,"q") is never Hurwitz, and PSL(5,"q") is Hurwitz if and only if "q" = 7⁴ or "q" = "p"^"n"_"p", harv|Tamburini|Vsemirnov|2006.

Similarly, many groups of Lie type are Hurwitz. The finite classical groups of large rank are Hurwitz, harv|Lucchini|Tamburini|1999. The exceptional Lie groups of type G2 and the Ree groups of type 2G2 are nearly always Hurwitz, harv|Malle|1990. Other families of exceptional and twisted Lie groups of low rank are shown to be Hurwitz in harv|Malle|1995.

There are 12 sporadic groups that can be generated as Hurwitz groups: the Janko groups J₁, J₂ and J₄, the Fischer groups Fi₂₂ and Fi'₂₄, the Rudvalis group, the Held group, the Thompson group, the Harada-Norton group,the third Conway group Co₃, the Lyons group and best of all, the Monster, harv|Wilson|2001.

ee also

*(2,3,7) triangle group

Notes

References

*cite journal|last = Hurwitz|first = A.|title = Über algebraische Gebilde mit Eindeutigen Transformationen in sich|journal = Mathematische Annalen|volume = 41|year = 1893|pages = 403–442|doi = 10.1007/BF01443420
* | year=1999 | journal=Journal of Algebra | issn=0021-8693 | volume=219 | issue=2 | pages=531–546
* | year=2000 | journal=Journal of the London Mathematical Society. Second Series | issn=0024-6107 | volume=61 | issue=1 | pages=81–92
* | year=1990 | journal=Canadian Mathematical Bulletin | issn=0008-4395 | volume=33 | issue=3 | pages=349–357
* | year=1995 | volume=207 | chapter=Small rank exceptional Hurwitz groups | pages=173–183
* | year=2006 | journal=Journal of Algebra | issn=0021-8693 | volume=300 | issue=1 | pages=339–362
*citation | last=Wilson | first=Robert A. | title=The Monster is a Hurwitz group | journal=J. Group Theory | volume=4 | year=2001 | number=4 | pages=367&ndash;374 | id=MathSciNet|id=1859175 | doi=10.1515/jgth.2001.027