# Isomorphism between R^inf and a proper subset of R^inf

1. Jan 8, 2009

### jojo12345

I'm fairly certain the following is a vector space isomorphism $$\phi :\mathbb{R}^\infty\rightarrow\mathbb{R}^\infty$$ where the vector space is the space of infinite sequences of real numbers and phi is defined by $$\phi(a_1,a_2,...)=(0,a_1,a_2,...)$$. The mapping is linear and the inverse seems to be well defined. Is my logic flawed?

2. Jan 8, 2009

### Hurkyl

Staff Emeritus
What's the inverse of (1, 0, 0, 0, ...)?

(Why are you asking about the inverse anyways?)

3. Jan 8, 2009

### jojo12345

You're right. As I wrote it, phi isn't an isomorphism. I'll try again. Define $$\phi :\mathbb{R}^\infty\rightarrow S$$ ,where $$S\subset \mathbb{R}^\infty$$ is the subspace containing all infinite sequences of reals of the form $$(0,a_1,a_2,...)$$, as: $$\phi(a_1,a_2,...)=(0,a_1,a_2,...)$$. Define the inverse function $$\phi^{-1}:S\rightarrow\mathbb{R}^\infty$$ as: $$\phi^{-1}(0,a_1,a_2,...)=(a_1,a_2,...)$$. Does this establish an isomorphism between $$\mathbb{R}^\infty$$ and $$S$$?

4. Jan 8, 2009

### Hurkyl

Staff Emeritus
Let's see... the things you need are
1. phi is well-defined
2. phi is linear
3. phi^-1 is well-defined
4. phi^-1 is linear
5. The composition of phi with phi^-1 is the identity
6. The composition of phi^-1 with phi is the identity

You've checked all of those, right? Then congratulations, you have an isomorphism!

Incidentally, you can prove a more general theorem: if phi is injective can you show it's an isomorphism onto its image? (And thus, all you need for your problem is an injection that is not surjective)

5. Jan 8, 2009

### jojo12345

yes, it must be because it's surjective onto its image. Also, I think that 1,2,3,5,6 combined imply 4. Thanks a lot!

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?