We have the following statement (Matrix analysis and applied linear algebra, Mayer)
The range of every linear function $f:\mathbb{R}^n\rightarrow\mathbb{R}^m$ is a subspace of $\mathbb{R}^m$, and every subspace of $\mathbb{R}^m$ is the range of some linear function.
I understand the proof of the first part, but I need to understand the argument in the second part of statement.
He starts as follows.
Let $V$ be a subspace of $\mathbb{R}^m$. Suppose that $\{v_1, v_2, \dots, v_n\}$ is spanning set for $V$ so that $$V = \{\alpha_1v_1+ \dots + \alpha_nv_n \mid \alpha_i \in \mathbb{R}\}.$$
I know that every vector space has spanning set. Clearly, if the spanning set of $V$ has $k$ vectors, then there is also a spanning set of $V$ which has $k+1$ vector.
But I do not know why he is sure that the spanning set of $V$ has $n$ vectors. What am I missing? Thank you.
Best Answer
The statement is not very clear in this way. There are two true things.
The second statement does not specify the domain of the function, and, for reasons of dimension, it has to be $\mathbb{R}^n$ with $n \geq m$. A better statement would be the following
For any $n \geq m$ and any subspace $V$ of $\mathbb{R}^m$, there is a linear function $f : \mathbb{R}^n \to \mathbb{R}^m$ such that $V$ is the range of $f$.
Then the proof is essentially as you say: take a spanning set $\{ v_1, \cdots, v_m \}$ of $V$, and define $$ f(e_i) = v_i, \quad i = 1, 2, \dots, m, $$ and $$ f(e_i) = v_m, \quad i = m+1, \dots, n. $$