Prove $f(x)=Cx$ for All $x$: Functional Equation

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In summary, the conversation discusses the proof that if a function $f(x)$ is continuous for all $x$ and satisfies the property $f(a+b) = f(a) + f(b)$ for all $a$ and $b$, then it must be of the form $f(x) = Cx$, where $C = f(1)$. The speaker has shown this to be true for all rational numbers and is trying to use the continuity of $f$ to prove it for all real numbers. The use of convergent sequences and the density of rationals in $\mathbb{R}$ is mentioned as a potential approach. Finally, it is noted that without the continuity requirement, $f$ can either be of the
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alexmahone
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Suppose $f(x)$ is continuous for all $x$ and $f(a+b)=f(a)+f(b)$ for all $a$ and $b$. Prove that $f(x)=Cx$, where $C=f(1)$.

I have shown that $f(x)=Cx$ for all rational numbers. How do I use the continuity of $f$ to show it is true for all $x$?
 
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  • #2
Alexmahone said:
I have shown that $f(x)=Cx$ for all rational numbers.
Having this, remember that rationals are dense in $\mathbb R.$
 
  • #3
Krizalid said:
Having this, remember that rationals are dense in $\mathbb R.$

Intuitively, I can see that it must be true but I'm having trouble proving it.
 
  • #4
Alexmahone said:
Intuitively, I can see that it must be true but I'm having trouble proving it.
Every real number is the limit of a sequence of rational numbers.
The function is continuous. What continuity and convergent sequences?
 
  • #5
Plato said:
Every real number is the limit of a sequence of rational numbers.
The function is continuous. What continuity and convergent sequences?

Got it. Thanks!
 
  • #6
By the way, if you do not include the requirement that the function be continuous, all f are either of the form f(x)= cx or the graph of y= f(x) is dense in the plane.
 

FAQ: Prove $f(x)=Cx$ for All $x$: Functional Equation

What is a functional equation?

A functional equation is an equation where the unknown is a function, rather than a variable or a constant. It involves finding a function that satisfies the given equation, rather than solving for a specific value.

How do you prove a functional equation?

The most common approach to proving a functional equation is to use mathematical induction. This involves proving that the equation holds for a base case, and then showing that if it holds for one value, it also holds for the next value. This process is repeated until it can be shown that the equation holds for all values.

What is the specific functional equation "Prove $f(x)=Cx$ for All $x$"?

The specific functional equation "Prove $f(x)=Cx$ for All $x$" is a statement that the function $f(x)$ is equal to some constant $C$ times the input $x$ for all possible values of $x$. In other words, the output of the function is always a multiple of the input.

What is the significance of proving $f(x)=Cx$ for All $x$?

Proving that a function is equal to some constant times its input for all possible values is significant because it allows us to make generalizations about the function without having to consider individual values. This can be useful in many areas of mathematics and science, such as in optimization problems and modeling real-world phenomena.

Can the functional equation "Prove $f(x)=Cx$ for All $x$" be proven for all types of functions?

Yes, the functional equation "Prove $f(x)=Cx$ for All $x$" can be proven for all types of functions. This is because it is a general statement about functions and their relationship to their inputs. The proof may vary depending on the specific type of function, but the concept remains the same.

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