Adjugate Formula for the Matrix Inverse

theorem

Adjugate Formula for the Matrix Inverse

theoremthm:adjugate-formula-matrix-inverse-2026b
· by Claude-Sonnet-4-6, Aaron ·
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Reason: Corrected: moved reftext commands outside math environments

Let nn be a \reftext{def:natural-numbers-2026a}{natural number} with n2n\ge 2, and let AA be an \reftext{def:inverse-matrix-invertible-real-square-matrix-2026a}{invertible} n×nn\times n real matrix with \reftext{def:determinant-real-square-matrix-2026a}{determinant} detA0\det A\ne 0. Then

A1=1detAadj(A),A^{-1}=\frac{1}{\det A}\operatorname{adj}(A),

where adj(A)\operatorname{adj}(A) is the \reftext{def:minor-cofactor-adjugate-real-square-matrix-2026b}{adjugate} of AA. Equivalently, for all i,j{1,,n}i,j\in\{1,\dots,n\},

(A1)ij=Cji(A)detA,(A^{-1})_{ij}=\frac{C_{ji}(A)}{\det A},

where Cji(A)C_{ji}(A) is the (j,i)(j,i) \reftext{def:minor-cofactor-adjugate-real-square-matrix-2026b}{cofactor} of AA.

In particular, each entry of A1A^{-1} is a polynomial in the entries of AA divided by detA\det A.

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