Analysis: Is f(x) Continuous at Every Number?

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In summary, the statement "If the function f: (0,1)--> R is continuous in every irrational number x then f is continuous at every number" is false. This is proven by a counterexample, Thomae's function, which is continuous at every irrational number but discontinuous at all rational numbers. However, there are functions, such as f(x) = x, that are continuous at every irrational number and at every number. Further discussion and modifications of Thomae's function can be found in a paper by Dr. Beanland.
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AlexHall
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I need to show if the following is true or false.

If the function f: (0,1)--> R is continuous in every irrational number x then f is continuous at every number.

Thank you
 
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The answer is false: Define f by f(x) = 0 if x is irrational; and f(x) = 1/q where x = p/q, p coprime to q. This function is continuous at every irrational and discontinuous at all the rationals.
 
  • #3
e(ho0n3 said:
The answer is false: Define f by f(x) = 0 if x is irrational; and f(x) = 1/q where x = p/q, p coprime to q. This function is continuous at every irrational and discontinuous at all the rationals.

There are functions where this is true. f(x) = x is continuous at every irrational number on [0,1], and is continuous at every number. However, this is not generally the case:
I believe Thomae's function serves as a counterexample to the statement which you need to disprove which is what e(ho0on3 mentioned.

There is an interesting paper on Thomae's function by Dr. Beanland, discussing how to modify the function so that is differentiable
<www.people.vcu.edu/~kbeanland/Papers/ThomaesFunction.pdf>
Very interesting, he mentions some way to quantify irrationalness which is the part of the paper that goes over my head, but is interesting nonetheless
 

1. What is the definition of continuity?

Continuity is a property of a function where it is unbroken or uninterrupted over a certain interval or domain. In other words, a function is continuous if there are no abrupt changes or gaps in its graph.

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

To determine if a function is continuous at a specific point, you need to check if the limit of the function as it approaches that point is equal to the value of the function at that point. If the limit and the function value are the same, then the function is continuous at that point.

3. What are the three types of continuity?

The three types of continuity are point continuity, left-sided continuity, and right-sided continuity. Point continuity means that a function is continuous at a specific point. Left-sided continuity means that a function is continuous on the left side of a point. Right-sided continuity means that a function is continuous on the right side of a point.

4. Can a function be continuous at one point but not at another?

Yes, a function can be continuous at one point but not at another. This is because continuity is a local property, meaning it only applies to a specific point. A function can be continuous at one point but have a discontinuity at another point.

5. How does the concept of continuity relate to differentiability?

Differentiability is a stronger condition than continuity. A function is differentiable at a point if it is continuous at that point and has a defined derivative. This means that all differentiable functions are also continuous, but not all continuous functions are differentiable.

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