Can two different functions have an infinite number of solutions?

1. Jun 17, 2013

Tim_B

Let f(x) and g(x) be non-piecewise defined functions that are defined for all real numbers. Furthermore, let f(x) and g(x) be continuous and differentiable at all points.
Are there two functions f(x) and g(x) such that f(x)=g(x) for all points over some interval (a,b], and f(x)g(x) for all points over some interval (b,c)? Assume a≠b and b≠c.
Basically, what I'm asking is this: can two different functions equal one another for all points over some interval? If I'm not making myself clear, see the attached picture:
https://www.physicsforums.com/attachment.php?attachmentid=59652&stc=1&d=1371520767
Thanks for your help. PS: This is my first post.

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2. Jun 17, 2013

Office_Shredder

Staff Emeritus
Non-piecewise defined isn't a very clearly defined term. For example is |x| allowed? What about f(x) = x if x > 0, and -x if x<0?

3. Jun 17, 2013

Simon Bridge

Welcome to PF;
... yes they can. Unless you tighten your definition to the point where they can't.

4. Jun 17, 2013

Tim_B

I realize that, but |x| isn't differentiable over its entire domain anyway.

5. Jun 17, 2013

Tim_B

Could you give an example or two?

6. Jun 17, 2013

Office_Shredder

Staff Emeritus
Yeah but |x3| is.

Whether it's differentiable or not is totally irrelevant anyway. Your question as posed doesn't make sense - saying "not piecewise defined" is not a well-defined statement.

7. Jun 18, 2013

HallsofIvy

Staff Emeritus
Once you have said "differentiable for all x" you don't need "not piecewise defined".

No, the fact that two functions are equal at every point on an interval does not mean they are equal for other points.

8. Jun 18, 2013

Tim_B

Good point. I should redefine my terms.

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