Is the Composition of Even and Odd Functions Always Even?

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Discussion Overview

The discussion revolves around the properties of the composition of even and odd functions, specifically whether the composition of two functions results in an even function under certain conditions. Participants explore various scenarios involving even and odd functions, aiming to understand the implications of these properties in mathematical proofs.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants inquire whether the composition of an even function with another function results in an even function, and under what conditions this holds true.
  • Others provide examples of even and odd functions, such as the identity function and the absolute value function, to illustrate their points.
  • Several participants suggest using definitions of even and odd functions to prove the properties of their compositions, emphasizing the need for a structured proof approach.
  • There is a discussion about specific cases, such as when g is even and f is odd, and whether the composition results in an even function.
  • Some participants express uncertainty about their understanding of the properties and seek clarification on the definitions and implications of the compositions.
  • One participant questions the notation and terminology used in mathematical expressions, highlighting a potential misunderstanding in the reading of the equality sign.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the properties of the composition of even and odd functions, with multiple competing views and uncertainties remaining throughout the discussion.

Contextual Notes

Limitations include the need for clear definitions and proofs, as well as unresolved mathematical steps in the exploration of function compositions.

roam
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Considering the composition of two functions ƒ · g

If g is even then does this mean that ƒ · g is even? why?

Or if g is odd and ƒ is even, then ƒ · g is even?

How can we show these statements?

Thanks.
 
Last edited:
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Consider the fact that the identity function I(x) = x is odd and the absolute value function A(x)=|x| is even.
 
Just like you would do any proof of this sort: use the definitions.

If g is even, then g(-x)= g(x). Now, what can you say of f\cdot g(-x)= f(g(-x))?

If g is odd then g(-x)= -g(x). If f is even then f(-x)= f(x). Now, what can you say of f\cdot g(-x)= f(g(-x))?
 
If g is even, then g(-x)= g(x). Now, what can you say of f\cdot g(-x)= f(g(-x))?

Could you explain a little bit more on this part please? Thanks.

f(-x)= f(x)
g(-x)= g(x)

f.g(-x) = f(-x(g(-x)))
f.g(-x) = f(g(x))

It is even? Because a function is even if:
f:(-a,a) -> R if for all x \in (-a,a), f(x) = f(-x)

Please help me, I don't know if I'm right.

Regards,
 
Last edited:
Hi roam! :smile:

You need (for each part) a proof that starts "(ƒ · g)(-x) = … ", and finishes " … = (ƒ · g)(x)."

Hint: suppose g(3) = 7.

If g is even, what are (ƒ · g)(3) and (ƒ · g)(-3)?

If g is odd, what are (ƒ · g)(3) and (ƒ · g)(-3)? :smile:
 
If g is even, what are (ƒ · g)(3) and (ƒ · g)(-3)? even

If g is odd, what are (ƒ · g)(3) and (ƒ · g)(-3)? even

What if g is odd and f is even? would the (f · g) be even?
 
Hi roam! :smile:

No … you're missing the point!

Follow the hint … if g(3) = 7, what is (ƒ · g)(3) (how is (ƒ · g)(3) defined? and so what is it)? And what is (ƒ · g)(-3) (same procedure)?
 
roam said:
Could you explain a little bit more on this part please? Thanks.

f(-x)= f(x)
g(-x)= g(x)

f.g(-x) = f(-x(g(-x)))
NO! f.g(-x)= f(g(-x)) as you say below:
f.g(-x) = f(g(x))
and f(g(x))= f.g (x) doesn't it?

It is even? Because a function is even if:
f:(-a,a) -> R if for all x \in (-a,a), f(x) = f(-x)
So you have just said, (f.g)(-x)= f.g(x), haven't you?

Please help me, I don't know if I'm right.

Regards,
 
HallsofIvy said:
and f(g(x))= f.g (x) doesn't it?

Sorry for the off-topic:

Is the "=" sign typically read as "is equal to" or as "equals"...?
I thought it would be the former but according to your question tag "doesn't it" you seem to use the latter.
Hm, probably both are possible:smile:
 
  • #10
Actually, I debated whether to say "doesn't it" or "isn't it" myself! Yes, "=" can be read as either "equals" or "is equal to".

I was thinking "f(g(x) equals f.g(x) doesn't it" but I considered "f(g(x)) is equal to f.g(x) isn't it".
 

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