Abstract
Let \(\mathbb{R}^{m \times n}\) be the vector space of \(m \times n\) real matrices, and let \(\phi\colon \mathbb{R}^{m \times n} \longrightarrow \mathbb{R}^{m \times n}\) be a linear transformation such that \(\operatorname{vol}(\phi(A)) = \operatorname{vol}(A)\) for all \(A \in \mathbb{R}^{m \times n}\) . If \(m \neq n\) , then there exist two orthogonal matrices \(P \in \mathbb{R}^{m \times m}\) and \(Q \in \mathbb{R}^{n \times n}\) such that \(\phi(A) = P A Q\) for all \(A \in \mathbb{R}^{m \times n}\) . If \(m = n\) , then there exist two orthogonal matrices \(P \in \mathbb{R}^{n \times n}\) and \(Q \in \mathbb{R}^{n \times n}\) such that either \(\phi(A) = P A Q\) for all \(A \in \mathbb{R}^{n \times n}\) or \(\phi(A) = P A^{\mathrm T} Q\) for all \(A \in \mathbb{R}^{n \times n}\) .