Equivalence theorem for nonsingular matrices: the linear transformation given by T(x)=Ax is an isomorphism.
Theorem: An $n \times n$ matrix $A$ is nonsingular iff $T(x) = Ax$ is an isomorphism. An isomorphism is a bijective linear transformation, and for finite-dimensional spaces of the same dimension, injectivity, surjectivity, and bijectivity are all equivalent.