Single Point Continuity - Spivak Ch.6 Q5

In summary, the conversation discusses the concept of continuity and how it can be defined and negated. Examples are given, such as a function that is continuous only at a single point and a function that is continuous through sequences. The open set definition of continuity is also mentioned. It is pointed out that division by 0 is not allowed in the Math section.
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  • #2
Miguel said:
Hey Guys, I posed this on Math Stackexchange but no one is offering a good answering. I though you guys might be able to help :)

https://math.stackexchange.com/questions/3049661/single-point-continuity-spivak-ch-6-q5
The idea is: No matter how close (##\delta##) you get to ##x=a##, there is always a positive distance (##\varepsilon##) such that there are image points (##f(a),f(x)##) at least so far from another.

Can you formally negate the definition of continuity, i.e. formally define what it means to be discontinuous at ##a\,?##
 
  • #3
@Miguel, in future posts, please do not delete the homework template. Its use is required here for homework questions.
 
  • #4
There are many variations of this problem, another one is

##f(x) = \left\{\begin{array}{c}x,\hspace{20pt}x\in\mathbb{Q}\\-x,\hspace{20pt}x\in\mathbb{R}\setminus\mathbb{Q}\end{array}\right.##

which is continuous only at ##x=0##.
 
  • #5
@Miguel: There is also a notion of continuity through sequences and their convergence*: f is continuous at x iff ##( x_n \rightarrow x) \rightarrow (f(x_n) \rightarrow f(x)) ## Can you see what happens as you approach a Rational or Irrational through a sequence? What happens when x=a? You may also want to consider the open set definition: Consider an open set in the target space (ban open interval). What is its inverse image under this map. Is ihe inverse image open **?*This is not valid for all spaces, but it is for this one
**This is really not the pointwise definition, but I think it is a nice exercise.

EDIT: Miguel: We don't allow division by 0 in the Math section. You must get an upvote quickly to avoid the 1/0 in your Avatar ;). .
 
Last edited:

1. What is single point continuity?

Single point continuity, also known as pointwise continuity, is a property of a function where the limit of the function exists at a specific point in its domain. This means that the function is continuous at a single point, but may not necessarily be continuous at other points.

2. How is single point continuity different from uniform continuity?

Uniform continuity is a stronger form of continuity that requires the function to be continuous at every point in its domain. In contrast, single point continuity only requires the function to be continuous at a specific point in its domain.

3. Can a function be uniformly continuous but not single point continuous?

Yes, it is possible for a function to be uniformly continuous but not single point continuous. This can occur when the function has a discontinuity at a single point, but the discontinuity is "small enough" that it does not affect the overall continuity of the function.

4. How is single point continuity related to the epsilon-delta definition of continuity?

The epsilon-delta definition of continuity is a mathematical way of describing the concept of single point continuity. It states that a function is continuous at a point if, for any small positive number (epsilon), there exists a small positive number (delta) such that the difference between the function's output at any point within delta distance of the input point is less than epsilon.

5. Can a function have multiple points of single point continuity?

Yes, a function can have multiple points of single point continuity. This means that the function is continuous at each of these points, but may not be continuous at other points in its domain.

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