# Homework Help: Linear Algebra subspace troubles

1. Oct 11, 2009

### Servarus

1. The problem statement, all variables and given/known data
Let V be a finite dimensional subspace. Let V$$\supseteq$$U1$$\supseteq$$U2$$\supseteq$$...$$\supseteq$$Uk. Show that there exists k such that Uk=Uk+1=...=Un=...

2. Relevant equations
We were also told to assume none of the subspaces are zero dimensional, and to think about how the dimensions can change throughout.

3. The attempt at a solution
I know that all the Ui's are closed under vector addition, but I really don't know what to do with the information. I really don't know where to start. Any and all help will be appreciated.

Thanks guys.

2. Oct 12, 2009

### Servarus

Ok, I believe I figured it out after a couple hours.
First I must assume that Uk is not zero dimensional. Then I say that another subspace must be zero dimensional since V is finite dimensional. Then prove that Uk is that subspace.

Let me know if I am doing this correctly. Thanks in advance.

3. Oct 12, 2009

### HallsofIvy

Do you realize you just said "assume Ui is not zero dimensional" and then said "prove that Uk is that (zero dimensional) subspace"? So you are going to prove something you are assuming is false!

In any case, you said in your first post that "We were also told to assume none of the subspaces are zero dimensional" so why are you focusing on zero dimensional subspaces?

It is NOT necessary, and you were told to assume it didn't happen, that NONE of the given subspaces is 0 dimensional

For example, suppose V= R3, U1= {(x, y, 0)}, U2= {(x, 0, 0)} and, if k> 1, Uk= {(x, 0, 0)}. There are no zero dimensional subspaces in that sequence.

What you can do, since the dimensions of all subspaces must be positive, is let "k" be the smallest dimension of any of the subspaces. Any set of positive integers contains a smallest integer.

4. Oct 12, 2009

### Servarus

Oh wow, I'm extremely sorry. Been up for over 24 hours doing homework and obviously not thinking correctly.

Now I realized that I do need to make k the smallest dimension of any of the subspaces, and then be able to prove that k=k+1=k+2=...=n=... Also, I'm thinking I would show this by showing that the span{Uk}=span{Uk+1}=... But I'm a little stumped on how I would go about doing this.

5. Oct 12, 2009

### HallsofIvy

If $U_i[itex] is NOT equal to [itex]U_{i+1}$, then, since $U_{i+1}\subset U_i$, $U_{i+1}$ must has lower dimension than $U_i$. What is the smallest possible reduction in dimension? What is the maximum number of times that can happen?

6. Oct 12, 2009

### Servarus

So the proof would basically go as follows:
1) Assume $U_i$ $\neq$ $U_{i+1}$.
2) Then do a proof by contradiction and show that $U_i$ must equal $U_{i+1}$ because $U_i$ is the lowest dimensional subspace.