Simultaneous Linear Equations has Unique Solution iff Rank of Matrix of Coefficients equals Number of Columns

Theorem
Let $S$ be a system of $m$ simultaneous linear equations in $n$ variables:


 * $\ds \forall i \in \set {1, 2, \ldots, m} : \sum_{j \mathop = 1}^n \alpha_{i j} x_j = \beta_i$

Let $S$ be expressed in matrix form as:


 * $\mathbf A \mathbf x = \mathbf b$

where:


 * $\mathbf A = \begin {pmatrix}

\alpha_{1 1} & \alpha_{1 2} & \cdots & \alpha_{1 n} \\ \alpha_{2 1} & \alpha_{2 2} & \cdots & \alpha_{2 n} \\ \vdots & \vdots & \ddots & \vdots \\ \alpha_{m 1} & \alpha_{m 2} & \cdots & \alpha_{m n} \\ \end {pmatrix}$, $\mathbf x = \begin {pmatrix} x_1 \\ x_2 \\ \vdots \\ x_n \end{pmatrix}$, $\mathbf b = \begin {pmatrix} \beta_1 \\ \beta_2 \\ \vdots \\ \beta_m \end {pmatrix}$

Then $S$ has exactly one solution :
 * $\map \rho {\mathbf A} = n$

where $\map \rho {\mathbf A}$ denotes the rank of $\mathbf A$.