# Dual-output Function?

1. Nov 23, 2008

### scikidus

"Dual-output" Function?

This isn't homework: I am experimenting with factorization.

Does anyone know of a function f(x) which for some value of x returns one value for f(x), but for every other value of x returns some other value?

Example: I'm trying to find a function f(x), where

x = 0, f(x) = 1
x != 0, f(x) = 0

My function is only dealing with non-negative integers, if that helps.

I've already derived a function that does this, but it uses absolute values, which is a nuisance.

Anyone know of anything like this?

For reference, here is my formula:

$$f(x) =\frac{1-\frac{\left|2x-1\right|}{2x-1}}{2} = \frac{\left|4x-2\right| - 4x-2}{8x-4}$$

Last edited: Nov 23, 2008
2. Nov 23, 2008

### Vid

3. Nov 23, 2008

### CRGreathouse

Re: "Dual-output" Function?

That's the characteristic function of zero (Sloane's A000007). But your post seemed to focus on giving it a closed form. Why?

4. Nov 24, 2008

### HallsofIvy

Staff Emeritus
Re: "Dual-output" Function?

What exactly do you want? When you say "f(0)= 1, f(x)= 0 if x is not 0" you have already defined a function. And if you want a closed form, why is absolute value a "nuisance"?

5. Nov 24, 2008

### scikidus

Re: "Dual-output" Function?

Thanks to those who have responded, you've helped a lot.

@Vid: The link you supplied helped me solve another problme I was working with, so thanks.

@CRGreathouse: That, too, is of great help. I wasn't sure if 0^0 would be considered defined, but that greatly simplifies my procedure.

@HallsofIvy: Yes, that techincally defines a function, but I was looking for a mathematical equation which would supply that result. Two ways are the one I supplies in ym OP, and f(x) = 0^x.

Also, I called absolute value a nuisance because I am build an equation and I need to then invert it. If there are absolute values, then things get very tricky, by which I mean impossible to solve.

6. Nov 24, 2008

### Office_Shredder

Staff Emeritus
Re: "Dual-output" Function?

It's interesting you decided f(x)=0^x is an equation that gives this, since 0^x has to be defined at x=0 separately anyway, so you haven't really gained anything. And there's no way in hell you're going to invert this sucker

7. Nov 24, 2008

### CRGreathouse

Re: "Dual-output" Function?

This function (regardless of whether you view it as a closed-form equation or not) can't be inverted. f(9) = f(3) = 0, so what would f^-1(0) be?

8. Nov 24, 2008

### scikidus

Re: "Dual-output" Function?

Terribly sorry, I misspoke/posted. By "invert," I meant not the function, but the final equation that I'm working on.

9. Nov 25, 2008

### CRGreathouse

Re: "Dual-output" Function?

Yes, but inverting the final equation will involve inverting that special function.

10. Nov 25, 2008

### CRGreathouse

Re: "Dual-output" Function?

Actually, I've decided that using the characteristic function of 0 essentially allows you to build piecewise functions, so perhaps you can simply invert piecewise.

11. Nov 25, 2008

### Ben Niehoff

Re: "Dual-output" Function?

If your function only needs to be defined over the integers, you can try

$$f(n) = \frac{\sin \pi n}{n}$$