# I need a function

1. Nov 30, 2004

### Chen

Hi,

I am looking for a non-linear function f(x) such that f'(x1) = f'(x2) but x1 != x2.

Thanks,
Chen

2. Nov 30, 2004

### Chen

I guess sin(x) will do... :)

3. Nov 30, 2004

### Chen

Although I wouldn't mind hearing other suggestions, I'm sure there are lots of functions that satisfy this requirement.

Thanks

4. Nov 30, 2004

### jcsd

Any function that is not injective has the quality that f(x_1) = f(x_2) does not imply that x_1 = x_2.

edited to add: oops I see he little ' now,, in this case you just need it so the derivative of the function is not injective.

So quickly f'(x) = 3x^2 is not injective and f(x) = x^3 is non-linear.

Last edited: Nov 30, 2004
5. Nov 30, 2004

### Gokul43201

Staff Emeritus
Here's a couple :
1) any periodic function,
2) any polynomial higher than a quadratic

6. Dec 1, 2004

### Chen

Hmm, sorry I think I forgot to mention one thing. I tried to simplify the task, which is to find a non-linear function, so that its tangent line at (x1, f(x1)) is the same tangent line at (x2, f(x2)). So in that perspective, x^3 doesn't work.

Thanks :)