Definition: We have $\left|A \right| \le \left| B \right|$ iff exist $f: A \to B $ is injection. And $\left|A \right| = \left| B \right|$ iff exist $f: A \to B$ is bijection.
My question: if $\left|A \right| \le \left| B \right|$ and $\left|A \right| \ge \left| B \right|$ then $\left|A \right| = \left| B \right|$.
Thanks for help me.