# Isomorphic between plane and line

1. Aug 21, 2009

### hermitian

Hi,

I understand that the open interval (0,1) is isomorphic to the real line $$\mathbb{R}^1$$

May i know whether there is also isomorphism from $$\mathbb{R}^2$$ to $$\mathbb{R}^1$$

thanks a lot!!!!

2. Aug 22, 2009

### Dragonfall

What do you mean by an isomorphism in this case?

3. Aug 22, 2009

### hermitian

hmm.... the usual sense of isomorphism.... where there is a structure preserving bijective map from R^2 to R^1. sorry is my question still vague?

4. Aug 22, 2009

### Hurkyl

Staff Emeritus
The question is which structure are you interested in! It's structure as a set? As an additive group? As a topological space?

I imagine it's the topology you care about. In that case, the answer is "no". The common proof (AFAIK) is to consider the image of a subset of R2 in the shape of the letter Y -- if such an isomorphism existed, it must be injective on the Y. Can you derive a contradiction from that?

5. Aug 22, 2009

### g_edgar

There is a Borel isomorphism between $\mathbb{R}^2$ and $\mathbb{R}$.

There is a group somorphism between $\mathbb{R}^2$ and $\mathbb{R}$ (both with addition).

BUT: There is no map between $\mathbb{R}^2$ and $\mathbb{R}$ that is both a Borel isomorphism and a group isomorphism.