感觉这本书的一些定义并不严谨

第6版135页，"A matrix that is not invertible is sometimes called a singular matrix, and an invertible matrix is called a nonsingular matrix." 这里根本就没有说到这个 matrix 得是 square matrix 的前提，相比之下 《Elementary Linear Algebra 》给的定义要清晰严谨得多。