# Embedding open connected Riemann Surfaces in $\mathbb{C}^2$

This question arises in the context of a question asked on MSE: Are concrete Riemann surfaces Riemann domains over $$\mathbb{C}$$. Part of the answer to that question is the question above which is being asked here since it appears to be at a "research level".

Is there an example (or even an obstruction) to embedding an open connected Riemann surface into $$\mathbb{C}^2$$?

A theorem of Narasimhan (Narasimhan, R. "Imbedding of open Riemann surfaces", Göttingen Nachrichten, No. 7 (1960), pp. 159-165; also see American Journal of Mathematics Vol. 82, No. 4 (Oct., 1960), pp. 917-934) proves that any open connected Riemann surface has a non-singular embedding in $$\mathbb{C}^3$$.

Extending this to $$\mathbb{C}^2$$ seems to be difficult as a linear projection would introduce nodal singularities in general.

There are examples of affine algebraic curves that do not embed algebraically into $$\mathbb{A}^2$$. One obstruction in this case is that for a smooth curve $$X$$ in $$\mathbb{A}^2$$, we have $$\Omega^1_{X}$$ is a trivial line bundle, whereas there are affine algebraic curves with non-trivial $$\Omega^1_{X}$$. However, this is not an obstruction in the case of an open Riemann surface since line bundles on it are holomorphically trivial!

• By open, do you mean non-compact? – François Brunault Oct 30 '18 at 9:09
• @FrançoisBrunault. Yes. Sorry about that. Since I was talking "Riemann surfaces", I used terminology (perhaps old!) from that sub-area. – Kapil Oct 30 '18 at 12:26

This is an open problem, known as

Bell-Narasimhan conjecture. Every open Riemann surface admits a proper holomorphic embedding into $$\mathbb{C}^2$$.

Look at F. Forstnerič's book Stein Manifolds and Holomorphic Mappings, Problem 9.10.1 p. 446.

• Thanks for the reference. I thought it might be so! – Kapil Oct 30 '18 at 10:05

One of the first example of proper embedding of open Riemann surface is,

Proper embedding of open unit disk in $$\mathbb C^2$$ which is one of corollary of Fatou-Bieberbach domains (holomorphic dynamics in $$\mathbb C^2$$). There are results about proper embedding of annulus as well.

But general question is still open which is known as Bell-Narasimman conjecture.

Bell-Narasimman conjecture has two parts (both are open) -

1) Every open Riemann surface can be embedded in $$\mathbb C^2$$

2) Every embedded open Riemann surface can be properly embedded in $$\mathbb C^2$$.