Riemann's existence theorem

Template:Short description In mathematics, specifically complex analysis, Riemann's existence theorem states that the category of compact Riemann surfaces is equivalent to the category of complex complete algebraic curves.

Sometimes, the theorem also refers to a generalization (a theorem of Grauert–Remmert), which says that the category of finite topological coverings of a complex algebraic variety is equivalent to the category of finite étale coverings of the variety.

Original statement

Let $X$ be a compact Riemann surface, $p_1,\cdots ,p_s$ distinct points in $X$ and $a_1,\cdots ,a_s$ complex numbers. Then there is a meromorphic function $f$ on $X$ such that $f(p_i)=a_i$ for all $1\le i\le s$.

Proof

For now, see SGA 1, Expose XII, Théorème 5.1., or SGA 4, Expose XI. 4.3.

Consequences

By definition, if $X$ is a complex algebraic variety, the étale fundamental group of $X$ at a geometric point $x$ is the projective limit

${\pi }_1^{\text{et}}(X,x)=\underleftarrow{\lim }{\mathrm{Aut}}_X(Y)$

over all finite Galois coverings $Y$ of $X$. By the existence theorem, we have

${\mathrm{Aut}}_X(Y)={\mathrm{Aut}}_{X^{an}}(Y^{an})$.

Hence, ${\pi }_1^{\text{et}}(X,x)$ is exactly the profinite completion of the usual topological fundamental group ${\pi }_1(X^{\text{an}},x)$ of $X$ at $x$.

See also

• Algebraic geometry and analytic geometry

References

• Harbater, David. "Riemann’s existence theorem." The Legacy of Bernhard Riemann After 150 Years (2015) (ed. by L. Ji, F. Oort, S.-T. Yau), Beijing-Boston: Higher Education Press and International Press, ISBN 978-1571463180
• Ryan Patrick Catullo, Riemann Existence Theorem. A slide for the paper.
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• M. Artin, A. Grothendieck, J.-L. Verdier, SGA 4, Théorie des topos et cohomologie étale des schémas, 1963–1964, Tomes 1 à 3, Avec la participation de N. Bourbaki, P. Deligne, B. Saint-Donat, version : c46c8b4 2018-12-20 13:39:00 +0100
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• Remmert, Reinhold (1998), From Riemann surfaces to complex spaces, France, Paris: S´emin. Congr., 3, Soc. Math
• J. S. Milne (2008). Lectures on Étale Cohomology

External links

• Riemann's existence theorem (Mathoverflow)
• Finite Covers of Complex Varieties (Mathoverflow)
• Riemann's existence theorem (nLab)