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2010 Mathematics Subject Classification: Primary: 15A15 [MSN][ZBL]

The signed transposed matrix of cofactors for a given square matrix $A$. The $(i,j)$ entry of $\mathrm{adj}\,A$ is $$\mathrm{adj}\, A_{ij} = (-1)^{i+j} \det A(j,i)$$ where $A(j,i)$ is the minor formed by deleting the row and column through the matrix entry $A_{ji}$.

The expansion of the determinant in cofactors is expressed as $$A . \mathrm{adj}\, A = \mathrm{adj}\, A . A = (\det A) I$$ where $I$ is the identity matrix.

Some texts do not include the transposition in their definition. The term adjoint matrix is also used, but more commonly refers to the conjugate transpose of a matrix, see Adjoint matrix.

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