Algebraic and Analytic Geometry
《Algebraic and Analytic Geometry》 · 第 185 页 · Section 3.4「Summary—Elementary Row Operations and Rank」，定义段
A matrix is said to be in reduced row echelon form if the following three conditions are satisfied: (a) Any row containing a nonzero entry precedes any row in which all the entries are zero, if any; (b) the first nonzero entry in each row is the only nonzero entry in its column; (c) the first nonzero entry in each row is $1$ and it occurs in a column to the right of the first nonzero entry in the preceding row.