Linear Algebra/Subspaces and Spanning sets

One of the examples that led us to introduce the idea of a vector space was the solution set of a homogeneous system. For instance, we've seen in Example 1.4 such a space that is a planar subset of $$\mathbb{R}^3$$. There, the vector space $$\mathbb{R}^3$$ contains inside it another vector space, the plane.

Any vector space has a trivial subspace $$ \{\vec{0}\,\} $$. At the opposite extreme, any vector space has itself for a subspace. These two are the improper subspaces. Other subspaces are proper.

The next result says that Example 2.8 is prototypical. The only way that a subset can fail to be a subspace (if it is nonempty and the inherited operations are used) is if it isn't closed.

Briefly, the way that a subset gets to be a subspace is by being closed under linear combinations.

We usually show that a subset is a subspace with $$ (2)\implies (1) $$.

Parametrization is an easy technique, but it is important. We shall use it often.

No notation for the span is completely standard. The square brackets used here are common, but so are "$$\mbox{span}(S)$$" and "$$\mbox{sp}(S)$$".

The converse of the lemma holds: any subspace is the span of some set, because a subspace is obviously the span of the set of its members. Thus a subset of a vector space is a subspace if and only if it is a span. This fits the intuition that a good way to think of a vector space is as a collection in which linear combinations are sensible.

Taken together, Lemma 2.9 and Lemma 2.15 show that the span of a subset $$S$$ of a vector space is the smallest subspace containing all the members of $$S$$.

Since spans are subspaces, and we know that a good way to understand a subspace is to parametrize its description, we can try to understand a set's span in that way.

So far in this chapter we have seen that to study the properties of linear combinations, the right setting is a collection that is closed under these combinations. In the first subsection we introduced such collections, vector spaces, and we saw a great variety of examples. In this subsection we saw still more spaces, ones that happen to be subspaces of others. In all of the variety we've seen a commonality. Example 2.19 above brings it out: vector spaces and subspaces are best understood as a span, and especially as a span of a small number of vectors. The next section studies spanning sets that are minimal.

Exercises
/Solutions/