On algebraic and non-algebraic neighborhoods of rational curves

Abstract

We prove that for any $d>0$ there exists an embedding of the Riemann sphere ${\mathbb{P}}^1$ in a smooth complex surface, with self-intersection $d$, such that the germ of this embedding cannot be extended to an embedding in an algebraic surface but the field of germs of meromorphic functions along $C$ has transcendence degree $2$ over $\mathbb{C}$. We give two different constructions of such neighborhoods, either as blowdowns of a neighborhood of the smooth plane conic, or as ramified coverings of a neighborhood of a hyperplane section of a surface of minimal degree. The proofs of non-algebraicity of these neighborhoods are based on a classification, up to isomorphism, of algebraic germs of embeddings of ${\mathbb{P}}^1$, which is also obtained in the paper.

arXiv comment16 pages. In version 2 references and new results added. Version 3: statement of Lemma 4.7 corrected, a bibliographical reference added, minor editing