Continuous Functions: Does f(x+δ) = ε?

In summary: If you take a function like x2 and graph it on a coordinate plane, you'll see that the function is continuous at points where the x-axis and y-axis intersect. However, if you take the derivative of x2 at a point (x,y), you'll see that the derivative is discontinuous at points where the x-axis and y-axis intersect. This is because at these points, the y-value of the function is changing from point to point.
  • #1
Flying_Goat
16
0
A function defined on ℝ is continuous at x if given ε, there is a δ such that |f(x)-f(y)|<ε whenever |x-y|<δ. Does this imply that f(x+δ)-f(x)=ε? The definition only deals with open intervals so i am not sure about this. If this is not true could someone please show me a counter example for it?

Any help would be appreciated. Thanks.
 
Physics news on Phys.org
  • #2
No. You have to learn to think differently. Draw a lot of pictures and think about limiting processes, not equalities. A real function is continuous at x if I can draw a rectangular box around the point (x, f(x)), shrink the box arbitrarily small, and the function remains in the box. Consider f(x)=1 for all x. It is continuous everywhere, but for any epsilon>0 there is no delta which satisfies your statement. In fact, f(y)-f(x)=0 for all x,y, yet for any epsilon, no matter how small, I can choose delta arbitrarily large.
 
  • #3
Flying_Goat said:
A function defined on ℝ is continuous at x if given ε, there is a δ such that |f(x)-f(y)|<ε whenever |x-y|<δ. Does this imply that f(x+δ)-f(x)=ε? The definition only deals with open intervals so i am not sure about this. If this is not true could someone please show me a counter example for it?

Any help would be appreciated. Thanks.

Another approach is :

The description can also be interpreted as saying that one can find, for any ε>0, a value of δ>0 every point x in the interval:

(y-δ,y+δ) on the x-axis

Is mapped into the interval (f(y)-ε,f(y)+ε )

on the y-axis.

Try playing with relatively-simple functions like x2, and see what happens with
the expression f(x+δ)-f(x), for different values of δ, and how you can choose δ to make the difference be within ε.
 
  • #4
Thanks for your replies. I understand now.
 

1. What is a continuous function?

A continuous function is a mathematical function that has no sudden jumps or breaks in its graph. This means that the function can be drawn without lifting the pen from the paper.

2. How do you determine if a function is continuous?

A function is continuous if it satisfies the three conditions of continuity: 1) the function is defined at the point, 2) the limit of the function at the point exists, and 3) the limit equals the value of the function at the point.

3. What is the definition of the limit of a function?

The limit of a function at a point is the value that the function approaches as the input (x) gets closer and closer to the given point. It is denoted as lim f(x) = L as x approaches a.

4. How do you prove that a function is continuous?

To prove that a function is continuous, you need to show that it satisfies the three conditions of continuity and that the limit of the function at every 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 properties of continuous functions.

5. What is the epsilon-delta definition of continuity?

The epsilon-delta definition of continuity states that a function f(x) is continuous at a point x=a if for every positive value of epsilon, there exists a positive value of delta such that |f(x)-f(a)| < epsilon whenever |x-a| < delta. In other words, the function's output can be made as close as desired to its input by restricting the input to be within a certain distance from the given point.

Similar threads

Replies
9
Views
1K
  • Calculus
Replies
33
Views
3K
Replies
4
Views
2K
Replies
3
Views
1K
Replies
4
Views
967
  • Calculus
Replies
1
Views
954
Replies
36
Views
3K
  • Calculus and Beyond Homework Help
Replies
4
Views
2K
  • Calculus and Beyond Homework Help
Replies
6
Views
1K
Back
Top