Adjugate Matrix/Examples/Arbitrary Matrix 1

Example of Adjugate Matrix
Let $\mathbf A$ be the square matrix:


 * $\mathbf A = \begin {pmatrix} 1 & 1 & 0 & 0 \\ 0 & 1 & 1 & 0 \\ 0 & 0 & 1 & 1 \\ 0 & 0 & 0 & 1 \\ \end {pmatrix}$

Then the adjugate matrix of $\mathbf A$ is:


 * $\adj {\mathbf A} = \begin {pmatrix} 1 & -1 & 1 & -1 \\ 0 & 1 & -1 & 1 \\ 0 & 0 & 1 & -1 \\ 0 & 0 & 0 & 1 \\ \end {pmatrix}$

Proof
For a square matrix $\mathbf A = a_{i j}$ of order $4$, the adjugate matrix of $\mathbf A$ is:


 * $\adj {\mathbf A} = \begin {pmatrix} A_{11} & A_{21} & A_{31} & A_{41} \\ A_{12} & A_{22} & A_{32} & A_{42} \\ A_{13} & A_{23} & A_{33} & A_{43} \\ A_{14} & A_{24} & A_{34} & A_{44} \end {pmatrix}$

For each $a_{i j}$ in $\mathbf A$, we calculate the cofactors $A_{i j}$:

Hence:
 * $\adj {\mathbf A} = \begin {pmatrix} 1 & -1 & 1 & -1 \\ 0 & 1 & -1 & 1 \\ 0 & 0 & 1 & -1 \\ 0 & 0 & 0 & 1 \\ \end {pmatrix}$

We check this result, using Product of Matrix with Adjugate equals Determinant by Unit Matrix.

First we note that $\map \det {\mathbf A} = 1$, by Determinant of Upper Triangular Matrix.

and all is well.