# Math Help - proof

1. ## proof

Let A={a_1,a_2,a_3,...} be a countably infinite set, and let A'=A-{a_1}. Prove that |A|=|A'|.

2. Originally Posted by natewalker205
Let A={a_1,a_2,a_3,...} be a countably infinite set, and let A'=A-{a_1}. Prove that |A|=|A'|.
Hint: $f:A \to A',f(a_i) = a_{i+1}$ is a bijection.

3. ## onto

i can't get the onto part of the bijection

4. Originally Posted by natewalker205
i can't get the onto part of the bijection
Let $a_k \in A'$, $a_{k-1} \in A$.
This is meaningful because k > 1.