Hurwitz quaternion order

Summary

The Hurwitz quaternion order is a specific order in a quaternion algebra over a suitable number field. The order is of particular importance in Riemann surface theory, in connection with surfaces with maximal symmetry, namely the Hurwitz surfaces.[1] The Hurwitz quaternion order was studied in 1967 by Goro Shimura,[2] but first explicitly described by Noam Elkies in 1998.[3] For an alternative use of the term, see Hurwitz quaternion (both usages are current in the literature).

Definition edit

Let   be the maximal real subfield of    where   is a 7th-primitive root of unity. The ring of integers of   is  , where the element   can be identified with the positive real  . Let   be the quaternion algebra, or symbol algebra

 

so that   and   in   Also let   and  . Let

 

Then   is a maximal order of  , described explicitly by Noam Elkies.[4]

Module structure edit

The order   is also generated by elements

 

and

 

In fact, the order is a free  -module over the basis  . Here the generators satisfy the relations

 

which descend to the appropriate relations in the (2,3,7) triangle group, after quotienting by the center.

Principal congruence subgroups edit

The principal congruence subgroup defined by an ideal   is by definition the group

 mod  

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).

Application edit

The order was used by Katz, Schaps, and Vishne[5] to construct a family of Hurwitz surfaces satisfying an asymptotic lower bound for the systole:   where g is the genus, improving an earlier result of Peter Buser and Peter Sarnak;[6] see systoles of surfaces.

See also edit

References edit

  1. ^ Vogeler, Roger (2003), On the geometry of Hurwitz surfaces (PhD), Florida State University.
  2. ^ Shimura, Goro (1967), "Construction of class fields and zeta functions of algebraic curves", Annals of Mathematics, Second Series, 85 (1): 58–159, doi:10.2307/1970526, JSTOR 1970526, MR 0204426.
  3. ^ Elkies, Noam D. (1998), "Shimura curve computations", Algorithmic number theory (Portland, OR, 1998), Lecture Notes in Computer Science, vol. 1423, Berlin: Springer-Verlag, pp. 1–47, arXiv:math.NT/0005160, doi:10.1007/BFb0054850, MR 1726059.
  4. ^ Elkies, Noam D. (1999), "The Klein quartic in number theory" (PDF), in Levi, Sylvio (ed.), The Eightfold Way: The Beauty of Klein's Quartic Curve, Mathematical Sciences Research Institute publications, vol. 35, Cambridge University Press, pp. 51–101, MR 1722413.
  5. ^ Katz, Mikhail G.; Schaps, Mary; Vishne, Uzi (2007), "Logarithmic growth of systole of arithmetic Riemann surfaces along congruence subgroups", Journal of Differential Geometry, 76 (3): 399–422, arXiv:math.DG/0505007, doi:10.4310/jdg/1180135693, MR 2331526, S2CID 18152345.
  6. ^ Buser, P.; Sarnak, P. (1994), "On the period matrix of a Riemann surface of large genus", Inventiones Mathematicae, 117 (1): 27–56, Bibcode:1994InMat.117...27B, doi:10.1007/BF01232233, MR 1269424, S2CID 116904696. With an appendix by J. H. Conway and N. J. A. Sloane.{{citation}}: CS1 maint: postscript (link)