There is this question, but the definition of "even" and "odd" that I am using uses integers instead of just natural numbers; i.e.,
- An integer $n$ is even iff there is some integer $k$ such that $n=2k$.
- An integer $n$ is odd iff there is some integer $k$ such that $n=2k+1$.
Here is what I have so far:
First we show that an integer $n$ is even or odd.
We first use induction on the positive integers.
For the base case, $1=2\cdot0+1$ so we are done.
Now suppose inductively that $n$ is even or odd.
If $n$ is even, then $n=2k$ for some $k$ so that $n+1=2k+1$ (odd).
If $n$ is odd, then $n=2k+1$ for some $k$ so that $n+1=2(k+1)$ (even).
This closes the induction, so every $n\in\mathbf{Z}^+$ is even or odd.
Now we show every $n\in\mathbf{Z}^-$ is even or odd.
Let $n\in\mathbf{Z}^-$.
Then $n=-k$ for some $k\in\mathbf{Z}$ (I think this follows immediately from most definitions of the integers.).
Suppose $k$ is even.
Then $k=2j$ for some $j$ so that $n=-k=-2j=2(-j)$ (even).
Now suppose $k$ is odd.
Then $k=2j+1$ for some $j$ so that $n=-k=-(2j+1)=-2j-1=-2j-1+1-1=-2j-2+1=2(-j-1)+1$ (odd).
For $0$, note that $0=2\cdot0$ (even).
Now we show that $n\in\mathbf{Z}$ cannot be both even and odd.
Suppose for the sake of contradiction that $n\in\mathbf{Z}$ is both even and odd.
Then there are integers $k,j$ such that $n=2k=2j+1$.
This implies that $2(k-j)=1$ (like in the referenced question).
So we must show that $1$ cannot be even in order to complete the proof.
This is where I am having trouble.
I know that if I let $f:\mathbf{Z}\to\mathbf{Z};x\mapsto2x$ be a function, then $f$ is increasing so since $f(0)=0$ and $f(1)=2$ and $0<1<2$, there is no integer $m$ such that $f(m)=1$.
But this seems complicated so I was wondering if there was an easier way to do this.
So my real question is: how can I show that $1$ is not even?
(This is not homework.)
Best Answer
To show that $1$ is not even:
I assume you can prove or accept that $a \cdot 0 = 0$ for all $a \in \mathbb{Z}$, the product of a positive and negative number is negative, and $2a > a$ when $a >0$.
If $1$ is even, then there must exists $a < 1$ such that $2a = 1$. However, the only $a < 1$, which is an integer, is $0$ and clearly $2 \cdot 0 = 0$. So $1$ can not be be written as $2 \cdot a$ for any $a \in \mathbb{Z}$. $1$ is odd.