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Separable gonality of a Gorenstein curve. (English) Zbl 0921.14014
From the paper: An integral projective curve $$X$$, which may have singular points, of arithmetic genus $$g\geq 2$$ is said to be hyperelliptic if there is a finite morphism $$X\to\mathbb{P}^1$$ of degree 2. However, there is a phenomenon which never happens in the case of smooth hyperelliptic curves; that is, the degree-two morphism may be inseparable. A hyperelliptic curve with this property is said to be of inseparable type. – On the other hand, any (singular or nonsingular) hyperelliptic curve is Gorenstein, i.e. the dualizing sheaf of the curve is invertible. The purpose of this short note is to give a characterization of hyperelliptic curves of inseparable type in the category of Gorenstein curves in terms of the separable gonality of a curve, which is defined to be the smallest possible degree of a finite separable morphism from the curve to the projective line.
Main result: Let $$X$$ be a Gorenstein curve of genus $$g\geq 2$$. Then $$k_s(X)\leq g+1$$. Furthermore, equality occurs if and only if $$X$$ is hyperelliptic curve of inseparable type.

##### MSC:
 14H20 Singularities of curves, local rings 14M05 Varieties defined by ring conditions (factorial, Cohen-Macaulay, seminormal)