ThmDex – An index of mathematical definitions, results, and conjectures.
Columns of a real matrix need not span the whole space
Formulation 0
Let $A \in \mathbb{R}^{2 \times 2}$ be a D4571: Real matrix such that
(i) $a_1, a_2 \in \mathbb{R}^{2 \times 1}$ are each a D5200: Real column matrix
(ii) \begin{equation} A = \begin{bmatrix} a_1 & a_2 \end{bmatrix} = \begin{bmatrix} 1 & 1 \\ 1 & 1 \end{bmatrix} \end{equation}
Let $x \in \mathbb{R}^{2 \times 1}$ be a D5200: Real column matrix such that
(i) \begin{equation} x = \begin{bmatrix} 1 \\ 0 \end{bmatrix} \end{equation}
Then \begin{equation} \forall \, r_1, r_2 \in \mathbb{R} : x = \begin{bmatrix} 1 \\ 0 \end{bmatrix} \neq \begin{bmatrix} r_1 + r_2 \\ r_1 + r_2 \end{bmatrix} = r_1 \cdot a_1 + r_2 \cdot a_2 \end{equation}
Proofs
Proof 0
Let $A \in \mathbb{R}^{2 \times 2}$ be a D4571: Real matrix such that
(i) $a_1, a_2 \in \mathbb{R}^{2 \times 1}$ are each a D5200: Real column matrix
(ii) \begin{equation} A = \begin{bmatrix} a_1 & a_2 \end{bmatrix} = \begin{bmatrix} 1 & 1 \\ 1 & 1 \end{bmatrix} \end{equation}
Let $x \in \mathbb{R}^{2 \times 1}$ be a D5200: Real column matrix such that
(i) \begin{equation} x = \begin{bmatrix} 1 \\ 0 \end{bmatrix} \end{equation}
Clear. $\square$