1. ## Denumerable Set

My book shows that the set of integers, $\displaystyle \mathbb{Z},$ is denumerable by expressing the elements of the set as infinite sequence

$\displaystyle \mathbb{Z}=\{0, 1,-1,2,-2,...\}$, and then shows that the function

$\displaystyle f: \mathbb{N}\rightarrow \mathbb{Z}$ is bijective, where

$\displaystyle f$ is defined as

$\displaystyle f(n)=\frac{1+(-1)^n(2n-1)}{4}$

The picture shows that there is one-one correspondence, such as this:

$\displaystyle 1 \rightarrow 0$

$\displaystyle 1 \rightarrow 1$

$\displaystyle 2 \rightarrow -1$

$\displaystyle 3 \rightarrow 2$

$\displaystyle 4 \rightarrow -2$

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Now I want to prove that $\displaystyle f(n)=\frac{1+(-1)^n(2n-1)}{4}$ bijective, but $\displaystyle (-1)^n$ in the expression is making it very clumsy.

To prove the one-to-one isn't too bad, since $\displaystyle f(a_1)=f(a_2)$ seems to look clean, but now when I get to the part where I need to prove it an onto function, I got a huge mess, so I am wondering whether it be alright to split the proof in two parts such that

$\displaystyle f(n) = g(s) \cup h(t)$ and define $\displaystyle g(s) \subset \mathbb{Z}^-$ and $\displaystyle h(t) \subset \mathbb{Z}^+$ and by so doing, I plan to prove that

there exists an odd positive integer $\displaystyle p\in \mathbb{N}$ such that $\displaystyle g(s)=\frac{1+(-1)^s(2s-1)}{4}$ where $\displaystyle s=2n-1, n \in \mathbb{N}$

and there also exists an even positive integer $\displaystyle q\in \mathbb{N}$ such that $\displaystyle h(t)=\frac{1+(-1)^t(2t-1)}{4}$ where $\displaystyle t=2n, n \in \mathbb{N}$

Questions:

1. Is there a simpler way than to prove the onto-function in two parts?

2. Is there a better way with handling $\displaystyle (-1)^n$ expression ?

2. If $\displaystyle x=0$ then $\displaystyle f(1)=0$.
If $\displaystyle x>0$ then $\displaystyle f(2x)=x$.
If $\displaystyle x<0$ then $\displaystyle f(2|x|+1)=x$.
So it is onto.