Journal of the London Mathematical Society Advance Access originally published online on December 13, 2007
Journal of the London Mathematical Society 2008 77(2):287-298; doi:10.1112/jlms/jdm100
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© 2007 London Mathematical Society
A geometric characterization of orientation-reversing involutions
Departmento de Matemáticas
Fundamentales
Facultad de Ciencias
UNED
Madrid 28040
Spain
acosta@mat.uned.es
IGAT Institute, EPFL
Bâtiment BCH
CH-1015 Lausanne
Switzerland
We give a geometric characterization of compact Riemann surfaces admitting orientation–reversing involutions with fixed points. Such surfaces are generally called real surfaces and can be represented by real algebraic curves with non-empty real part. We show that there is a family of disjoint simple closed geodesics that intersect all geodesics of a pants decomposition at least twice in uniquely right angles if and only if such an involution exists. This implies that a surface is real if and only if there is a pants decomposition of the surface with all Fenchel–Nielsen twist parameters equal to 0 or
.
The first author was supported in part by MTM 2005-01637 and the second author was supported in part by SNFS grant number PBEL2-106180.
2000 Mathematics Subject Classification 30F10, 32G15 (primary), 14H50, 30F20 (secondary).
Received February 28, 2006; revised April 27, 2007; published online December 13, 2007.