TITLE: Can decimals/fractions be odd or even?
QUESTION [5 upvotes]: At school I asked the question and I kept wondering "Can fractions or decimals be odd or even?"

REPLY [3 votes]: There is a natural way to define "oddness" for fractions.  For integer $x$, let $\nu_{2}(x)$  denote the number of $2$'s that divide $x$.  For example, $\nu_2(6)=1, \nu_2(4)=2, \nu_2(12)=2, \nu_2(1)=0, \nu_2(7)=0$.  We can leave $\nu_2(0)$ undefined, or set it equal to $\infty$, as you like.
We can extend this to fractions via $$\nu_2\left(\frac{m}{n}\right)=\nu_2(m)-\nu_2(n)$$
This satisfies the lovely relation $$\nu_2(xy)=\nu_2(x)+\nu_2(y)$$
which holds even when $x,y$ are fractions.
With this tool in hand, we can define a number $x$ as "odd" if $\nu_2(x)=0$.  The product of two odd numbers is odd, while the product of an odd number and a non-odd number is non-odd.  However there is no natural definition of "even" numbers.  We could take  $\nu_2(x)\neq 0$ (but then the product of two even numbers might be odd), or $\nu_2(x)>0$ (but then we need a third term for $\nu_2(x)<0$).
See also a more comprehensive answer here.