Value of Adjugate of Determinant
Theorem
Let $D$ be the determinant of order $n$.
Let $D^*$ be the adjugate of $D$.
Then $D^* = D^{n - 1}$.
Proof
Let $\mathbf A = \begin{bmatrix} a_{11} & a_{12} & \cdots & a_{1n} \\
a_{21} & a_{22} & \cdots & a_{2n} \\
\vdots & \vdots & \ddots & \vdots \\
a_{n1} & a_{n2} & \cdots & a_{nn}\end{bmatrix}$ and $\mathbf A^* = \begin{bmatrix} A_{11} & A_{12} & \cdots & A_{1n} \\ A_{21} & A_{22} & \cdots & A_{2n} \\
\vdots & \vdots & \ddots & \vdots \\
A_{n1} & A_{n2} & \cdots & A_{nn}\end{bmatrix}$.
Thus:
- $\paren {\mathbf A^*}^\intercal = \begin{bmatrix} A_{11} & A_{21} & \cdots & A_{n1} \\
A_{12} & A_{22} & \cdots & A_{n2} \\
\vdots & \vdots & \ddots & \vdots \\
A_{1n} & A_{2n} & \cdots & A_{nn}\end{bmatrix}$
is the transpose of $\mathbf A^*$.
Let $c_{ij}$ be the typical element of $\mathbf A \paren {\mathbf A^*}^\intercal$.
Then by definition of matrix product:
- $\ds c_{ij} = \sum_{k \mathop = 1}^n a_{ik} A_{jk}$
Thus by the corollary of the Expansion Theorem for Determinants:
- $c_{ij} = \delta_{ij} D$
So by Determinant of Diagonal Matrix:
- $\map \det {\mathbf A \paren {\mathbf A^*}^\intercal} = \begin{vmatrix} D & 0 & \cdots & 0 \\
0 & D & \cdots & 0 \\
\vdots & \vdots & \ddots & \vdots \\
0 & 0 & \cdots & D\end{vmatrix} = D^n$
From Determinant of Matrix Product:
- $\map \det {\mathbf A} \map \det {\paren {\mathbf A^*}^\intercal} = \map \det {\mathbf A \paren {\mathbf A^*}^\intercal}$
From Determinant of Transpose:
- $\map \det {\paren {\mathbf A^*}^\intercal} = \map \det {\mathbf A^*}$
Thus as $D = \map \det {\mathbf A}$ and $D^* = \map \det {\mathbf A^*}$ it follows that:
- $DD^* = D^n$
Now if $D \ne 0$, the result follows.
However, if $D = 0$ we need to show that $D^* = 0$.
Let $D^* = \begin{vmatrix} A_{11} & A_{12} & \cdots & A_{1n} \\
A_{21} & A_{22} & \cdots & A_{2n} \\
\vdots & \vdots & \ddots & \vdots \\
A_{n1} & A_{n2} & \cdots & A_{nn}\end{vmatrix}$.
Suppose that at least one element of $\mathbf A$, say $a_{rs}$, is non-zero (otherwise the result follows immediately).
By Expansion Theorem for Determinants and its corollary, we can expand $D$ by row $r$, and get:
- $\ds D = 0 = \sum_{j \mathop = 1}^n A_{ij} t_j, \forall i = 1, 2, \ldots, n$
for all $t_1 = a_{r1}, t_2 = a_{r2}, \ldots, t_n = a_{rn}$.
But $t_s = a_{rs} \ne 0$.
So, by (work in progress):
- $D^* = \begin{vmatrix} A_{11} & A_{12} & \cdots & A_{1n} \\
A_{21} & A_{22} & \cdots & A_{2n} \\
\vdots & \vdots & \ddots & \vdots \\
A_{n1} & A_{n2} & \cdots & A_{nn}\end{vmatrix} = 0$
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