First Hurwitz triplet
In the mathematical theory of Riemann surfaces, the first Hurwitz triplet is a triple of distinct Hurwitz surfaces with the identical automorphism group of the lowest possible genus, namely 14 (genera 3 and 7 each admit a unique Hurwitz surface, respectively the Klein quartic and the Macbeath surface). The explanation for this phenomenon is arithmetic. Namely, in the ring of integers of the appropriate number field, the rational prime 13 splits as a product of three distinct prime ideals. The principal congruence subgroups defined by the triplet of primes produce Fuchsian groups corresponding to the triplet of Riemann surfaces.
Let be the real subfield of where is a 7th-primitive root of unity. The ring of integers of K is , where . Let be the quaternion algebra, or symbol algebra . Also Let and . Let . Then is a maximal order of (see Hurwitz quaternion order), described explicitly by Noam Elkies .
In order to construct the first Hurwitz triplet, consider the prime decomposition of 13 in , namely
where is invertible. Also consider the prime ideals generated by the non-invertible factors. The principal congruence subgroup defined by such a prime ideal I is by definition the group
namely, the group of elements of reduced norm 1 in equivalent to 1 modulo the ideal . The corresponding Fuchsian group is obtained as the image of the principal congruence subgroup under a representation to PSL(2,R).
Bound for systolic length and the systolic ratio
The Gauss–Bonnet theorem states that
thus we obtain that the area of these surfaces is
The lower bound on the systole as specified in , namely
Some specific details about each of the surfaces are presented in the following tables (the number of systolic loops is taken from ).The term Systolic Trace refers to the least reduced trace of an element in the corresponding subgroup . The systolic ratio is the ratio of the square of the systole to the area.
|Number of Systolic Loops||91|
|Number of Systolic Loops||78|
|Number of Systolic Loops||364|
- Elkies, N. (1999). The Klein quartic in number theory. The eightfold way. Math. Sci. Res. Inst. Publ. 35. Cambridge: Cambridge Univ. Press. pp. 51–101.
- Katz, M.; Schaps, M.; Vishne, U. (2007). "Logarithmic growth of systole of arithmetic Riemann surfaces along congruence subgroups". J. Differential Geom. 76: 399–422. arXiv: .
- Vogeler, R. (2003). "On the geometry of Hurwitz surfaces". Thesis. Florida State University.