# Definition:Determinant/Matrix/Examples

## Examples of Determinant

### Determinant of Order 1

This is the trivial case:

$\begin {vmatrix} a_{1 1} \end {vmatrix} = a_{1 1}$

Thus the determinant of an order $1$ matrix is that element itself.

### Determinant of Order 2

 $\displaystyle \begin {vmatrix} a_{1 1} & a_{1 2} \\ a_{2 1} & a_{2 2} \end{vmatrix}$ $=$ $\displaystyle \map \sgn {1, 2} a_{1 1} a_{2 2} + \map \sgn {2, 1} a_{1 2} a_{2 1}$ $\displaystyle$ $=$ $\displaystyle a_{1 1} a_{2 2} - a_{1 2} a_{2 1}$

### Determinant of Order 3

Let:

$\map \det {\mathbf A} = \begin {vmatrix} a_{1 1} & a_{1 2} & a_{1 3} \\ a_{2 1} & a_{2 2} & a_{2 3} \\ a_{3 1} & a_{3 2} & a_{3 3} \end {vmatrix}$

Then:

 $\displaystyle \map \det {\mathbf A}$ $=$ $\displaystyle a_{1 1} \begin {vmatrix} a_{2 2} & a_{2 3} \\ a_{3 2} & a_{3 3} \end {vmatrix} - a_{1 2} \begin {vmatrix} a_{2 1} & a_{2 3} \\ a_{3 1} & a_{3 3} \end {vmatrix} + a_{1 3} \begin {vmatrix} a_{2 1} & a_{2 2} \\ a_{3 1} & a_{3 2} \end{vmatrix}$ $\displaystyle$ $=$ $\displaystyle \map \sgn {1, 2, 3} a_{1 1} a_{2 2} a_{3 3}$ $\displaystyle$  $\, \displaystyle + \,$ $\displaystyle \map \sgn {1, 3, 2} a_{1 1} a_{2 3} a_{3 2}$ $\displaystyle$  $\, \displaystyle + \,$ $\displaystyle \map \sgn {2, 1, 3} a_{1 2} a_{2 1} a_{3 3}$ $\displaystyle$  $\, \displaystyle + \,$ $\displaystyle \map \sgn {2, 3, 1} a_{1 2} a_{2 3} a_{3 1}$ $\displaystyle$  $\, \displaystyle + \,$ $\displaystyle \map \sgn {3, 1, 2} a_{1 3} a_{2 1} a_{3 2}$ $\displaystyle$  $\, \displaystyle + \,$ $\displaystyle \map \sgn {3, 2, 1} a_{1 3} a_{2 2} a_{3 1}$ $\displaystyle$ $=$ $\displaystyle a_{1 1} a_{2 2} a_{3 3}$ $\displaystyle$  $\, \displaystyle - \,$ $\displaystyle a_{1 1} a_{2 3} a_{3 2}$ $\displaystyle$  $\, \displaystyle - \,$ $\displaystyle a_{1 2} a_{2 1} a_{3 3}$ $\displaystyle$  $\, \displaystyle + \,$ $\displaystyle a_{1 2} a_{2 3} a_{3 1}$ $\displaystyle$  $\, \displaystyle + \,$ $\displaystyle a_{1 3} a_{2 1} a_{3 2}$ $\displaystyle$  $\, \displaystyle - \,$ $\displaystyle a_{1 3} a_{2 2} a_{3 1}$

and thence in a single expression as:

$\displaystyle \map \det {\mathbf A} = \frac 1 6 \sum_{i \mathop = 1}^3 \sum_{j \mathop = 1}^3 \sum_{k \mathop = 1}^3 \sum_{r \mathop = 1}^3 \sum_{s \mathop = 1}^3 \sum_{t \mathop = 1}^3 \map \sgn {i, j, k} \map \sgn {r, s, t} a_{i r} a_{j s} a_{k t}$

where $\map \sgn {i, j, k}$ is the sign of the permutation $\tuple {i, j, k}$ of the set $\set {1, 2, 3}$.

The values of the various instances of $\map \sgn {\lambda_1, \lambda_2, \lambda_3}$ are obtained by applications of Parity of K-Cycle.