Functions, Domains, And equality

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Homework Help Overview

The discussion revolves around finding two different functions, f and g, both defined on the set of real numbers, that are equal for all rational inputs. Participants explore the implications of the problem statement and the nature of functions, particularly focusing on piecewise-defined functions.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • Some participants suggest defining functions that differ on irrational numbers while being equal on rational numbers. Others question the clarity of the problem statement regarding the equality of the functions across different domains.

Discussion Status

Participants are actively engaging with the problem, raising questions about the definitions and characteristics of functions. Some have proposed potential function definitions, while others express uncertainty about the problem's context and requirements.

Contextual Notes

There is mention of the original poster's background in precalculus and their familiarity with piecewise functions, which may influence their understanding of the problem. The discussion also touches on the appropriateness of the problem's placement within the precalculus curriculum.

L Huyghe
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Homework Statement



Given an example of two different functions f and g, both of which have the set of real numbers as their domain, such that f(x)=g(x) for every rational number.

2. The attempt at a solution

I have yet to figure a way to approach this problem. Since it appears as though they are only equal for rational numbers.
 
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It doesn't seem from the problem statement that they are only equal for rational numbers, but in that case, why not just use that in the function definitions? You could define a function that is 0 for all rational numbers and 1 at all irrational numbers, for example. Then come up with a second function that meets the problem criteria.
 
You are right in doesn't necessarily state that all they are not equal in terms of irrational numbers in their domain
 
L Huyghe said:

Homework Statement



Given an example of two different functions f and g, both of which have the set of real numbers as their domain, such that f(x)=g(x) for every rational number.

2. The attempt at a solution

I have yet to figure a way to approach this problem. Since it appears as though they are only equal for rational numbers.
That is the approach! Define f to be whatever you want, then define g to be that same function on the rationals, but something else on the irrationals. Remember that a "function" does not necessarily mean a single "formula". Such a function cannot be "continuous" anywhere. I do wonder why such a problem would be posted under "precalculus". Since you are the same person who posted the "find f(x) that maps (0, 1) to [0, 1]", what course are these for?
 
The problem does say that f and g are different. So they have to have a different value at one point, at least.
 
HallsofIvy said:
I do wonder why such a problem would be posted under "precalculus".
Don't people generally learn the basics of functions in a precalculus class, or earlier?

It sounds like he just did the chapter on piecewise-defined functions. (Tip to opening poster: information such as what lesson you have just learned can be useful both to give you ideas, and to help those who want to help you)
 
Sorry about that, It's probably a good idea to give some more background knowledge about myself. It's not for a course, Just reading precalculus in advance for next year, and I found these problems in the book. I know about piecewise function, but they have yet to be clearly defined in the book yet. These question are in the intro to functions section.

*Also I am still having trouble answering original question, It would be great if u could give me an example. Sorry If i was vague before.
 
[tex]f(x)=\left\{\begin{array}{cc}0,&\mbox{ if }<br /> x \in \matbb{Q} \\1, & \mbox{ otherwise }\end{array}\right.[/tex]

[tex]g(x)=\left\{\begin{array}{cc}0,&\mbox{ if }<br /> x \in \matbb{Q} \\14, & \mbox{ otherwise }\end{array}\right.[/tex]
 

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