# Applying Zorn's Lemma (Maximal Subspace)

1. Feb 6, 2010

### Discover85

1. The problem statement, all variables and given/known data
Suppose that V is a vector space, W and X are subspaces with X contained in W. Show that there is a subspace U of V which is maximal subject to the property that U intersect W equals X.

2. Relevant equations
N/A

3. The attempt at a solution
I know this uses Zorn's Lemma but I can't see how to apply it.

Thanks for any help in advance!

2. Feb 6, 2010

### rochfor1

So what do we need to apply Zorn's lemma? A partially ordered set in which every chain has an upper bound. Let's let our partially ordered set be the family of all subspaces of V whose intersections with W gives X ordered by inclusion. Given a chain in this poset, can you find a natural upper bound?

3. Feb 6, 2010

### Discover85

Would a natural upper bound for this poset be the subspace U such that U contains X and V-{W-X}?

Last edited: Feb 6, 2010
4. Feb 6, 2010

### rochfor1

We're trying to prove that U exists, so we can't use it as the upper bound here. Also, you don't have to find an upper bound for the whole poset, that's what Zorn's lemma gives. You just need to find an upper bound for an arbitrary chain in the poset. A subset S of our poset is a chain if $$A,B \in S$$ implies that either $$A\subseteq B$$ or $$B \subseteq A$$. What can you say about the union of a family of increasing subpsaces?

5. Feb 6, 2010

### Discover85

Wouldn't the union of increasing subspaces tend to some infinite subspace?

6. Feb 7, 2010

### rochfor1

Take out infinite (which is not necessarily true) and you've got it.

7. Feb 7, 2010