We will be concerned primarily with bijective proofs, i.e., showing that two sets have the same number of elements by exhibiting a bijection (one-to-one correspondence) between them.

• 我们主要关心的是对射的证明，也就是透过在两个集合间建立一个对射（一对一且映成的函数）来证明它们的元素个数相等。