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Theorem of Enriques-Petri type for a very ample invertible sheaf on a curve of genus three. (English) Zbl 0509.14030
A well known theorem of Noether-Enriques-Petri says that a canonical curve $$C$$ (i.e.: $$C\subseteq {\mathbb{P}}^{g-1}; g(C)=g; \omega_ C\simeq {\mathcal O}_ C(1))$$ is projectively normal and is intersection of quadrics unless $$C$$ is trigonal. Furthermore if C is trigonal the intersection of the quadrics through $$C$$ is a rational ruled surface or the Veronese surface [P. Griffiths and J. Harris, ”Principles of algebraic geometry” (1978; Zbl 0408.14001), p. 535]. In the paper under review the author studies a similar problem for curves of genus three embedded by complete linear series. Let L be a very ample line bundle of degree d on the curve X of genus three. Denote by $$\phi_ L(X)$$ the image of X in the embedding: $$\phi_ L:X\hookrightarrow {\mathbb{P}}(H^ 0(L)^{\nu})=:{\mathbb{P}}^ n.$$ Note that if $$d\geq 7$$ then $$\phi_ L(X)$$ is projectively normal [D. Mumford, CIME $$3^ o$$ Ciclo Varenna 1969, Quest. algebr. Varieties, 29-100 (1970; Zbl 0198.258)]. The cases $$d\leq 6$$ have been previously studied by the author [Tsukuba J. Math. 4, 269-279 (1980; Zbl 0473.14015)]. If $$d\geq 8$$ it follows from a theorem of B. Saint-Donat [C. R. Acad. Sci., Paris, Sér. A 274, 324-327 (1972; Zbl 0234.14012)] that $$\phi_ L(X)$$ is intersection of quadrics. So the only case left is $$d=7$$. In the first part of the paper, the author describes the scheme $$Q(\phi_ L(X))$$ defined by the ideal generated by the quadrics through $$\phi_ L(X)$$. The last section contains a comment on the general case. From the result of Saint-Donat quoted above it follows that if $$g(X)\geq 2$$ and $$\deg(L)=2g+1,$$ then the homogeneous ideal of $$\phi_ L(X)$$ can be generated by its elements of degree 2 and 3. The author makes the following conjecture: ”Assume $$X$$ non hyperelliptic then: (1) if $$h^ 0(L\otimes \omega_ X^{-1})=2, Q(\phi_ L(X))$$ is a rational ruled surface; (2) if $$h^ 0(L\otimes \omega_ X^{-1})=1, Q(\phi_ L(X))$$ is the union of $$\phi_ L(X)$$ and a line: (3) if $$h^ 0(L\otimes \omega_ X^{-1})=0, Q(\phi_ L(X))=\phi_ L(X).$$” - Section 1 proves the conjecture for $$g=3$$ and the paper ends with the proof of (1) for every $$g$$.
Reviewer: J.Brun

##### MSC:
 14H10 Families, moduli of curves (algebraic) 14F05 Sheaves, derived categories of sheaves, etc. (MSC2010) 14C20 Divisors, linear systems, invertible sheaves 14H45 Special algebraic curves and curves of low genus
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##### References:
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