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The separating semigroup of a real curve. (English. French summary) Zbl 1440.14271

Summary: We introduce the separating semigroup of a real algebraic curve of dividing type. The elements of this semigroup record the possible degrees of the covering maps obtained by restricting separating morphisms to the real part of the curve. We also introduce the hyperbolic semigroup which consists of elements of the separating semigroup arising from morphisms which are compositions of a linear projection with an embedding of the curve to some projective space.
We completely determine both semigroups in the case of maximal curves. We also prove that any embedding of a real curve of dividing type to projective space of sufficiently high degree is hyperbolic. Using these semigroups we show that the hyperbolicity locus of an embedded curve is in general not connected.

MSC:

14P25 Topology of real algebraic varieties
14H50 Plane and space curves
14H51 Special divisors on curves (gonality, Brill-Noether theory)
20M99 Semigroups

References:

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