# History of the connection between Riemann surfaces and complex algebraic curves

As noted in the question "Links between Riemann surfaces and algebraic geometry", there are strong connections between Riemann surfaces and algebraic geometry - for example, compact Riemann surfaces are equivalent to smooth complex algebraic curves.

I wondered if anyone could give insights into the history of this relationship - for example, which was studied first, who first realised the connection, did the theories develop concurrently for any period of time, etc.

Any individual insights, or links or book recommendations to find out more, would be greatly appreciated.

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The connection came from the paper by Dedekind and Weber "Theorie der algebraischen Functionen einer Veranderlichen", Crelle's Journal, 1882. In this paper the authors recover the theory by Riemann (including the famous Riemann-Roch theorem) by the abstract procedure of assigning a "curve" to an degree one transcendental field extension $\Omega/\mathbb{C}$ where "points" correspond to "places", i.e. discrete rank one valuations of the field $\Omega$. This theory inspired later further developments by Weil and Zariski and it may be considered the prehistory of the modern approach of abstract algebraic geometry.

This is also one of the first instances of modern structural mathematics based on (infinite) sets, much on the style later embraced by Bourbaki. The emphasis is on fields, valuations and ideals, the geometric counterpart of Dedekind's work in algebraic number theory. This point of view was not very well appreciated by some outstanding figures like Kronecker. This may be considered as a prelude of the controversy between formalists and intuitionists (Hilbert vs. Brouwer) that happened a few decades later.

There is an nice translation of Dedekind and Weber's paper by Stillwell published by the AMS, see AMS bookstore.

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Thank you very much for the insightful answer, and the link to the original work, which I always find it very good to be able to find. I do have one question - the answer seems to imply that algebraic curves, and algebraic geometry in general, was only connected to these areas much later - is that correct? –  Tait Jul 23 '14 at 20:04
More or less. While there was an algebraic tradition going through Hilbert, E. Noether, E. Artin, Krull and finally van der Waerden, Zariski found that the geometric arguments from the italian school were weak and, for instance, the classical theory of surfaces was not correctly developed. This was a motivation for writing a treatise on algebraic geometry, that never was finished because his energies were consumed by the classical text on Commutative Algebra with Samuel. In the mid fifties the sheaf revolution started with Cartan, Oka and finally Serre translated it to the algebraic setting. –  Leo Alonso Jul 24 '14 at 8:58

I have always felt this was due to Riemann himself: especially in: Theory of Abelian Functions, 1857. Of course the association of a Riemann surface to an algebraic curve is generally attributed to him, and there he also proves conversely there is a plane curve associated to a (compact) Riemann surface.

He proves there too the Riemann part of the Riemann Roch theorem both for a Riemann surface, and explains how to prove it algebraically for a plane curve. The Roch part, provided later, is merely the addition of the residue theorem to the Riemann theorem, to compute a period integral as a residue evaluation.

There are excellent translations of Riemann's works in a volume by Kendrick Press.

http://www.kendrickpress.com/Riemann.htm

for my account of Riemann's proof of his theorem, see pages 21-34 of:

http://www.math.uga.edu/%7Eroy/rrt.pdf

Let me clarify my use of the word "prove" in regard to Riemann's work. It is apparently true he made unjustified use of the Dirichlet principle in his argument that a compact Riemann surface has on it non zero entire differential forms and non constant meromorphic functions, and hence in his proof of the Riemann theorem for Riemann surfaces. However he explains how to simply write down such objects for plane curves, which by virtue of their representation allow the use of coordinate functions in this regard.

The purely algebraic work of others, is sometimes pointed to as filling a gap in the rigor of Riemann's arguments for Riemann - Roch, but in the case of curves already assumed to be algebraic, such as those associated to algebraic function fields, Riemann's arguments do not require the Dirichlet principle for the existence theorems, and hence are already completely rigorous. Purely algebraic formulations of Riemann Roch, although unnecessary to render Riemann's arguments rigorous in the case of algebraic function fields over the complex numbers, do however seem important for allowing the theory to be generalized to fields other than the complex numbers.

Briefly, the original Riemann Roch theorem over C, requires existence of g holomorphic differential forms where g is the topological genus, and the existence of meromorphic differentials of second kind associated to each point. Then the Riemann Roch formula for the number of meromorphic functions with given bounds on their poles is deduced from an argument via path integrals which are evaluated by Roch using residues. This original argument is completely rigorous for curves derived from algebraic functions fields over C. (I admit that although Riemann writes down the holomorphic forms on a plane curve, he only states that he could as easily write down the meromorphic ones, but does not do so.)

In the algebraic setting one has the existence of forms and functions essentially by hypothesis, and usually one defines the genus in terms of the space of entire forms, and deduces the Riemann Roch formula by algebraic means, avoiding the use of path integrals. In one approach, a judicious use of duality allows one to exploit just the holomorphic differentials and to deduce the existence of meromorphic differentials of second kind after the fact.

In the analytic case of a compact Riemann surface, in which the existence of either a non zero differential form or a non constant meromorphic function is non trivial, purely algebraic arguments do nothing to shore up the situation. So I feel the actual gaps in Riemann's work are on the analytic side, and were apparently filled in only later by those who supplied the analysis, such as Koebe, Hilbert, and Weyl.

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