# Concerning Subspaces of Infinite Dimensional Vector Spaces

1. Apr 19, 2012

### mathstew

I have a question concerning subspaces of infinite dimensional vector spaces. Specifically given any infinite dimensional vector space V, how might one construct an infinite decreasing chain of subspaces?

That is:

V=V0$\supseteq$V1$\supseteq$... , where each Vi is properly contained in Vi-1.

I know such chains must exist and I suspect that they should be easily constructed, however I am not familiar enough with infinite dimensional vector spaces to be confident with my attempts thus far.

Thanks for any help given!

2. Apr 19, 2012

### micromass

Staff Emeritus
Select a basis for the vector space. This should be finite, so we can select a countable chain $\{e_1,e_2,e_3,e_4,...\}$ of basis elements.

Then

$$span\{e_1\}\subseteq span\{e_1,e_2\}\subseteq span \{e_1,e_2,e_3\}\subseteq ...$$

is an infinite chain of subspaces.

3. Apr 19, 2012

### mathstew

Thanks!

So I suppose that with your notation:

$$V = span\{e_1,e_2,...\}\supseteq span\{e_2,e_3,...\}\supseteq...$$ will give me the infinite decreasing chain as desired.