# Basis of a subspace of a vectorspace

JamesTheBond
Is the basis for the subspace W of a vectorspace V spanned by the basis of the vectorspace V? If so how?

## Answers and Replies

Homework Helper
Is the basis for the subspace W of a vectorspace V spanned by the basis of the vectorspace V? If so how?

What exactly do you mean? A basis for V is a span for W.

Homework Helper
Arguably it is not, since elements in a basis of V are not in general going to be elements of W. W would certainly be in the span of a basis of V, but that is I would suggest subtley different from them being a spanning set for W.

Anyway, the original question is not very clear. There is no such thing as *the* basis of a vector space. And I don't know what it means for one basis to span a basis of subset.

If you mean: given W<V, and a basis set B for V, does B have a subset that is a basis for W, then the answer is no.

JamesTheBond
I am sorry, I misworded my question. At any rate, what I wanted to know was a confirmation of basis extension. For W < V, B_W (some basis of W) is linearly independent and spans W, therefore, there exists some B_V (basis of V) s.t. B_W < B_V.

I believe this is the basis extension theorem... not sure.

Homework Helper
For W < V, B_W (some basis of W) is linearly independent and spans W, therefore, there exists some B_V (basis of V) s.t. B_W < B_V.

I believe this is the basis extension theorem... not sure.

I'm not sure what B_W < B_V is supposed to mean, but if you meant that every linearly independent set in a vector space V (and hence a basis of a subspace W, too, since it's a linearly independent set) can be 'extended to the basis' of V, then the answer is yes.

JamesTheBond
Sorry, I meant to say: for some $$B_W \subset B_V$$ for some basis of V.

Homework Helper
And the answer, as radou said, is "yes".

Let {vi} be a basis for W. If it is also a basis for V we are done. If not, since it is clearly independent, it must not span V- there exist some v which is not a linear combination of {vi[/sup]}. Append v to that set of vectors and show that it is still a set of independent vectors. If it spans V, we are done. If not, repeat.

That works for a finite dimensional vector space. If you are working with an infinite dimensional vector space, you will have to use "transfinite" induction- use Zorn's lemma to prove that any collection of independent vectors in V (i.e. a basis for W) is contained in a basis for V.

Homework Helper
here is an example to think about. (1,0)n and (0,1) give a basis of R^2. now consider the subspace where y=x, i.e. the line at 45degrees through the origin. how woulkd you get a basis of that subspace from the given basis of R^2?