Proving Continuity of Functions on the Reals

In summary, f is a function defined on the reals that satisfies the functional equation f(x1+x2)=f(x1)+f(x2). If f is continuous at x=0, then it can be shown that f is continuous at all x in the reals by using the definition of continuity and the knowledge that f(-x) = -f(x). This can be done by considering the limit of f(x + epsilon) - f(x) and using the fact that f(a+ h) = f(a) + f(h).
  • #1
Math_Geek
23
0

Homework Statement


Prove: If f is defined on the reals and continuous at x=0, and if f(x1+x2)=f(x1)+f(x2) for all x1,x2 in the reals, then f is continuous at all x in the reals.

Homework Equations



Using defn of limits and continuity

The Attempt at a Solution


is this like proving that the sum of two functions is continuous? I am a bit confused, this is the last on of the homework, then I can enjoy my easter.
 
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  • #2
Math_Geek said:
is this like proving that the sum of two functions is continuous? I am a bit confused, this is the last on of the homework, then I can enjoy my easter.
No. f is a function that satisfies the functional equation f(x_1 + x_2) = f(x_1) + f(x_2). You're told that f is continuous at 0, and you're supposed to use this to conclude that f is continuous everywhere.
 
  • #3
so show since f is continuous at 0, there exists an epsilion >0 such that and deal with the two functions?
 
  • #4
There's just one function here: f. You want to prove that it's continuous (on all of R).
 
  • #5
ok so how does the f(x1)+f(x2) play into it?
 
  • #6
You also know something else. f is continuous at 0.

Now use the definition of continuity and these two facts. You might also find it helpful to prove that f(-x) = -f(x) [hint: f(0)=0].
 
  • #7
Math_Geek said:
ok so how does the f(x1)+f(x2) play into it?

Hint: what is f(x + epsilon) - f(x)? :smile:
 
  • #8
[tex]\lim_{x\rightarrow a} f(x)= \lim_{h\rightarrow 0}f(a+ h)[/tex]
where h= x- a. Then use the fact that f(a+ h)= f(a)+ f(h).
 

What is continuity of a function on the reals?

Continuity of a function on the reals means that the function has no sudden jumps or breaks and can be drawn without lifting the pen from the paper. In other words, the function has a smooth and continuous graph.

How is continuity of a function on the reals proven mathematically?

To prove continuity of a function on the reals, we need to show that the limit of the function at a certain point is equal to the value of the function at that point. This can be done using the epsilon-delta definition of continuity or by using the intermediate value theorem.

What is the difference between continuity and differentiability of a function on the reals?

Continuity and differentiability are related but different concepts. A function is continuous if it has no sudden jumps or breaks, while a function is differentiable if it has a well-defined derivative at every point. A function can be continuous but not differentiable, but if a function is differentiable, it must also be continuous.

How does the continuity of a function on the reals affect its graph?

The continuity of a function on the reals ensures that its graph is smooth and has no gaps or sharp turns. This means that the function can be drawn without lifting the pen from the paper. If a function is not continuous, its graph may have breaks or jumps, making it difficult to draw.

What are some real-world applications of proving continuity of functions on the reals?

The concept of continuity of functions on the reals is important in many fields, such as physics, engineering, and economics. It is used to model real-world phenomena and make predictions based on mathematical models. For example, the continuity of a function representing the motion of an object can help predict its future position and velocity.

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