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Tool to compute an Adjoint Matrix (Adjugate) for a square matrix, name given to the transpose of the cofactors matrix.

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Tag(s) : Mathematics, Algebra, Symbolic Computation

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Tool to compute an Adjoint Matrix (Adjugate) for a square matrix, name given to the transpose of the cofactors matrix.

### How to compute the adjugate matrix?

To calculate the adjoint matrix $${\rm Adj}$$ of the square matrix $$M$$, compute $$^{\operatorname t}{\rm Cof}$$ the transpose of the cofactors matrix of $$M$$.

$${\rm Adj}=^{\operatorname t}{\rm Cof}$$

To calculate the cofactors matrix $${\rm Cof}(M)$$, compute, for each value of the matrix in position $$(i,j)$$, the determinant of the associated sub-matrix $$SM$$ (called minor) and multiply with a $$-1$$ factor depending on the position in the matrix.

$${\rm Cof}_{i,j}=(-1)^{i+j}\text{Det}(SM_i)$$

To get the adjoint matrix, take the transposed of the calculated cofactor matrix.

Formula for a 2x2 matrix:

$$M = \begin{pmatrix} a & b \\ c & d \end{pmatrix}$$

$${\rm Cof}(M) = \begin{pmatrix} {{d}} & {{-c}} \\ {{-b}} & {{a}} \end{pmatrix}$$

$${\rm Adj}(M) = \begin{pmatrix} {{d}} & {{-b}} \\ {{-c}} & {{a}} \end{pmatrix}$$

Example: $$M = \begin{pmatrix} 4 & 3 \\ 2 & 1 \end{pmatrix} \Rightarrow {\rm Cof}(M) = \begin{pmatrix} {{1}} & {{-2}} \\ {{-3}} & {{4}} \end{pmatrix} \Rightarrow {\rm Comp}(M) = \begin{pmatrix} {{1}} & {{-3}} \\ {{-2}} & {{4}} \end{pmatrix}$$

Formula for a 3x3 matrix:

$$M = \begin{pmatrix} a & b & c \\d & e & f \\ g & h & i \end{pmatrix}$$

$${\rm Cof}(M) = \begin{pmatrix} +\begin{vmatrix} e & f \\ h & i \end{vmatrix} & -\begin{vmatrix} d & f \\ g & i \end{vmatrix} & +\begin{vmatrix} d & e \\ g & h \end{vmatrix} \\ & & \\ -\begin{vmatrix} b & c \\ h & i \end{vmatrix} & +\begin{vmatrix} a & c \\ g & i \end{vmatrix} & -\begin{vmatrix} a & b \\ g & h \end{vmatrix} \\ & & \\ +\begin{vmatrix} b & c \\ e & f \end{vmatrix} & -\begin{vmatrix} a & c \\ d & f \end{vmatrix} & +\begin{vmatrix} a & b \\ d & e \end{vmatrix} \end{pmatrix}$$

$${\rm Adj}(M) = \begin{pmatrix} +\begin{vmatrix} e & f \\ h & i \end{vmatrix} & -\begin{vmatrix} b & c \\ h & i \end{vmatrix} & +\begin{vmatrix} b & c \\ e & f \end{vmatrix} \\ & & \\ -\begin{vmatrix} d & f \\ g & i \end{vmatrix} & +\begin{vmatrix} a & c \\ g & i \end{vmatrix} & -\begin{vmatrix} a & c \\ d & f \end{vmatrix} \\ & & \\ +\begin{vmatrix} d & e \\ g & h \end{vmatrix} & -\begin{vmatrix} a & b \\ g & h \end{vmatrix} & +\begin{vmatrix} a & b \\ d & e \end{vmatrix} \end{pmatrix}$$