The necessary and sufficient conditions of similarity of two idempotent matrices are proved, the idempotency of linear combinations of two idempotent matrices is discussed.

摘证明两个同阶等阵相似的充条件是它们有相同的秩；给等阵的相似标准型；讨论两个等阵的线性组合仍是等阵的充条件。