TheoremBase

Adjugate Formula for the Matrix Inverse

A square real matrix of size at least 2 with nonzero determinant is invertible, and its inverse is the adjugate divided by the determinant.

Statement

In the setting of Routine Facts Used Without Citation: Arithmetic, Maps, Predicativity and Indices, let n∈Nn\in\mathbb{N} with 2≤n2\le n for the order of N0\mathbb{N}_{0} and let A∈Mn(R)A\in\mathcal{M}_{n}(\mathbb{R}) be a square real matrix with det⁡A≠0\det A\neq0, with entries of matrices, the determinant det⁡\det, invertible matrices and their inverses A−1A^{-1}, the cofactors Cij(A)C_{ij}(A) and the adjugate adj⁡(A)\operatorname{adj}(A), the scalar multiple of a matrix, and reciprocals and quotients as in Commutative Rings, Fields and Ordered Fields: Standard Notation (v).

AA is invertible, and A−1=(1/det⁡A)adj⁡(A)A^{-1}=(1/\det A)\operatorname{adj}(A).

(A−1)ij=Cji(A)/det⁡A(A^{-1})_{ij}=C_{ji}(A)/\det A for all i,j∈[n]i,j\in[n], A−1A^{-1} existing by clause formula.

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