Linear Algebra Done Right
《Linear Algebra Done Right》 · 第 91 页 · Section 3D「Invertibility and Isomorphic Vector Spaces」，Definition 3.80
A square matrix $A$ is called invertible if there is a square matrix $B$ of the same size such that $AB=BA=I$; we call $B$ the inverse of $A$ and denote it by $A^{-1}$. The terms nonsingular and singular mean the same as invertible and noninvertible, respectively.