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Find a Function

evinda

Well-known member
MHB Site Helper
Apr 13, 2013
3,720
Hi guys!!!I have a question..How can I show that the function that has the following identities:

[tex]\bullet[/tex] [tex]f(x)\neq 0[/tex] ,[tex]x\in\mathbb{R}[/tex].
[tex]\bullet[/tex] [tex]f(0)=f\left(\dfrac{2}{3}\right)[/tex].
[tex]\int_{0}^{x}\frac{f(t)f(x-t)}{f(\frac{2+t}{3})f(\frac{2+x-t}{3})}=\int_{0}^{x}\frac{f^2{(t)}}{f^2{(\frac{2+t}{3}})}[/tex]
is f(x)=c..??
That's what I did:

=>[tex]\int_{0}^{x}\frac{f(t)f(x-t)}{f(\frac{2+t}{3})f(\frac{2+x-t}{3})}=\int_{0}^{x}\frac{f^2{(t)}}{f^2{(\frac{2+t}{3}})}[/tex]

[tex]\frac{f(x)f(0)}{f(\frac{2+x}{3})f(\frac{2}{3})}[/tex]=[tex]\frac{f^2{(x)}}{f^2(\frac{2+x}{3})}[/tex] .

Because [tex]f(0)=f(\frac{2}{3})[/tex]

[tex]\frac{f(x)}{f(\frac{2+x}{3})}=\frac{f^{2}(x)}{f^2(\frac{2+x}{3}){}}[/tex]

=> [tex]f^2{(\frac{2+x}{3})}f(x)=f^2{(x)}f{(\frac{2+x}{3})}[/tex]

=>[tex]f{(\frac{2+x}{3})}=f(x)[/tex] , [tex]f(x) \neq 0[/tex]

=>f(x)=c

Is this right???? :confused:
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
8,780
Hi guys!!!I have a question..How can I show that the function that has the following identities:

[tex]\bullet[/tex] [tex]f(x)\neq 0[/tex] ,[tex]x\in\mathbb{R}[/tex].
[tex]\bullet[/tex] [tex]f(0)=f\left(\dfrac{2}{3}\right)[/tex].
[tex]\int_{0}^{x}\frac{f(t)f(x-t)}{f(\frac{2+t}{3})f(\frac{2+x-t}{3})}=\int_{0}^{x}\frac{f^2{(t)}}{f^2{(\frac{2+t}{3}})}[/tex]
is f(x)=c..??
That's what I did:

=>[tex]\int_{0}^{x}\frac{f(t)f(x-t)}{f(\frac{2+t}{3})f(\frac{2+x-t}{3})}=\int_{0}^{x}\frac{f^2{(t)}}{f^2{(\frac{2+t}{3}})}[/tex]

[tex]\frac{f(x)f(0)}{f(\frac{2+x}{3})f(\frac{2}{3})}[/tex]=[tex]\frac{f^2{(x)}}{f^2(\frac{2+x}{3})}[/tex] .

Because [tex]f(0)=f(\frac{2}{3})[/tex]

[tex]\frac{f(x)}{f(\frac{2+x}{3})}=\frac{f^{2}(x)}{f^2(\frac{2+x}{3}){}}[/tex]

=> [tex]f^2{(\frac{2+x}{3})}f(x)=f^2{(x)}f{(\frac{2+x}{3})}[/tex]

=>[tex]f{(\frac{2+x}{3})}=f(x)[/tex] , [tex]f(x) \neq 0[/tex]

=>f(x)=c

Is this right???? :confused:
Hi evinda!

It all looks good except for the last step.
(Oh, and you may want to add $dt$ to both integrals. ;))

For instance, since you did not specify that f is continuous, another solution would be:
$$f(x)=\begin{cases}
1 & \text{ if } \exists k \in \mathbb Z \text{ such that } x=1-3^k \\
2 & \text{ otherwise}
\end{cases}$$

With the additional condition that f is continuous, it does follow that $f(x)=C$.
This can be proven by picking a point with a different value and consider what that means for the limit where $x \to 1$ (proof by contradiction).
 
Last edited:

evinda

Well-known member
MHB Site Helper
Apr 13, 2013
3,720
Nice!!!Thank you veery much!!!! :p
 

johng

Well-known member
MHB Math Helper
Jan 25, 2013
236
Hi,
I can't quite figure out the hint. But here's a direct proof:
Let a and b be any reals with -1<a<1 and f a continuous function with f(ax+b)=f(x) for all x. Then f is a constant function.
Proof.
Let x be any real. Define the sequence xn inductively by x0 = x and for for n>0, xn=axn-1+b. Then an easy induction shows xn=anx+b(an-1)/(a-1). By hypothesis, f(x)=f(xn) for all n. So f(x)=limit as n approaches infinity f(xn)=f(-b/(a-1)); i.e. f is constant.
 

evinda

Well-known member
MHB Site Helper
Apr 13, 2013
3,720
Great..Thank you!!! :eek: