# 0, odd or even?

1. Apr 19, 2007

### repugno

Is 0 an odd or even number? The reason why I ask is this:

I need to write cosh(x) as the sum of an even and odd function. I could only come up with cosh(x) = cosh(x) + 0, where cosh(x) would be the even and 0odd. However, this doesn't make any sense since 0 is exactly divisible by 2 with no remainder, hence it is even. So which one is it?

2. Apr 19, 2007

### matt grime

0 is an even number.

But that has nothing to do with writing cosh as a sum of odd and even *functions*.

3. Apr 19, 2007

### HallsofIvy

Staff Emeritus
The definition of an "even" function is that f(-x)= f(x). The definition of "odd" function is f(-x)= -f(x). If f(x)= 0 for all x then f(-x)= 0= -0= -f(x) but also f(-x)= 0= f(x) so f(x)= 0, the constant function, is both even and odd.
However, as matt grime said, that has nothing to do with the fact that 0 = 2(0) is an even number.

cosh(x) is already an even function. sinh(x) is an odd function. In fact,
ex= cosh(x)+ sinh(x). cosh(x) and sinh(x) are the even and odd "parts" of ex.

4. Apr 20, 2007

### alastor

0 mod 2 = 0, it means that 0 is even

5. Apr 20, 2007

### Jimmy Snyder

In general, given a function f, you can write it as the sum of an even function and an odd function as follows:

$f_{even}(x) = (f(x) + f(-x))/2$
$f_{odd}(x) = (f(x) - f(-x))/2$

6. Apr 20, 2007

### mathwonk

what about 6? is it odd or even?

7. Apr 21, 2007

### HallsofIvy

Staff Emeritus
Okay, I'll bite: even?

Actually, a more interesting question would be whether 5 is odd or even.

The number 5 is obviously odd.

The constant function (which is what this thread is really about), f(x)= 5, is even.

8. Apr 22, 2007

### mathwonk

good point, so the answer to the OPs question is "yes".

i.e. all constant functions are even and one of them is also odd.