# Linearly independent functionals

Let $$f_1,\ldots,f_n$$ be linearly independent linear functionals on a vector space $$X$$. Show that there exist $$x_1,\ldots,x_n\in X,$$ such that the $$n\times n$$ matrix $$\big(f_i(x_j) \big)_{i,j=1}^n$$ is non-singular.

Let $F:X\to\mathbb R^n$ defined as follows $$F(x)=\big(f_1(x),\ldots,f_n(x)\big).$$ Clearly $F$ is a linear transformation and its range $Y$ is linear subspace of $\mathbb R^n$.
We shall show that the range is the whole $\mathbb R^n$. If it is not, then we can find a vector $v\in\mathbb R^n$, such that $v=(v_1,\ldots,v_n)\perp Y$, i.e. $$v\cdot y=0, \quad \text{for all}\,\,\,y\in Y.$$ But this means that for every $x\in X$: $$0=v\cdot F(x)=v\cdot \big(f_1(x),\ldots,f_n(x)\big)=v_1f_1(x)+\cdots+v_nf_n(x),$$ which means that the $f_1,\ldots,f_n$ are linearly dependent, a contradiction. Hence $F$ is onto. Let $x_1,\ldots,x_n$, such that $F(x_j)=e_j$, where $e_1,\ldots,e_n$ is the standard basis of $\mathbb R^n$. Then the matrix $$\big(f_i(x_j)\big)_{i,j=1}^n,$$ is the unit matrix, and its determinant is equal to $1$.
Note. The proof is valid even in the case in which $\mathbb R^n$ is replaced by $\mathbb C^n$.
• Is it possible to not use any inner product on $\mathbb{R}^n$? It seems to me that it's bad style to introduce an arbitrary inner product. Commented Jul 7, 2020 at 13:45
• How do you find the vector $v$? Commented Oct 24, 2021 at 6:23