Proving Thomae's Function f(x) Not Differentiable"

In summary, the Thomae function is defined as f(x)= p/q for rational numbers x= p/q with p and q relatively prime, and f(x)= 0 for irrational numbers x. It can be shown that this function is not differentiable at rational numbers, but the proof is more complicated and a specific sequence is needed to show that it is not differentiable at irrational numbers.
  • #1
limdaesung
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Thomae function f(x):(0,1)->R f(x)=p/q x is rational number (p and q are relatively prime natural number) f(x)=0 x in irrational number show that f is not differrentiable. l can show that this function is not differentiable at rational number. But i can't sequence that is example. not differrentiable at irrational number. how take this sequence.
 
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please i want to find this sequence quickly.
 
  • #3
A good start would be rereading the problem and then writing it down correctly! What you have written makes no sense.
Thomae function f(x):(0,1)->R f(x)=p/q x is rational number (p and q are relatively prime natural number) f(x)=0 x in irrational number

I presume that means that if is an irrational number then f(x)= 0, but it's not clear to me what f(x) is if x is rational. You say f(x)= p/q but you haven't told us what numbers p and q are.

I might be inclined to guess that you mean "if x is a rational number, x= p/q with p and q relatively prime, the f(x)= 1/q." That's a fairly well known function, often called the "modified Dirichlet function". (The regular Dirichlet function is f(x)= 0 if x is irrational, f(x)= 1 if x is rational.)

If that is the function you are talking about, it can be shown that lim(x->a) f(x)= 0 for all a so the modified Dirichlet function is continuous for all irrational a, discontinuous for rational a. Since a function cannot be differentiable where it is not continuous, if this is the right function, then I presume the problem is to show that this function, even though it is continuous at each irrational number, is not differentiable there.
 

1. What is Thomae's function f(x)?

Thomae's function f(x) is a mathematical function that evaluates to 0 if x is irrational and to 1/n if x is a rational number expressed in lowest terms p/q, where n is the denominator of the fraction.

2. Why is proving Thomae's function f(x) not differentiable important?

Proving that Thomae's function f(x) is not differentiable at any point is important because it is a counterexample to the common misconception that continuous functions are always differentiable. It also demonstrates the importance of carefully defining mathematical concepts and the limitations of certain mathematical concepts and tools.

3. How can you prove that Thomae's function f(x) is not differentiable?

To prove that Thomae's function f(x) is not differentiable, one can use the definition of differentiability, which requires the existence of a unique tangent line at each point on the function's graph. By showing that the function has sharp corners and no unique tangent line at rational points, it can be proven that the function is not differentiable.

4. What is the significance of the discontinuity of Thomae's function f(x)?

The discontinuity of Thomae's function f(x) at rational points is significant because it shows that the function is not continuous and therefore not differentiable at those points. This challenges the commonly held belief that continuous functions are always differentiable.

5. Are there any real-world applications of Thomae's function f(x)?

While Thomae's function f(x) may not have any direct real-world applications, it is a valuable mathematical concept that helps to illustrate the limitations of certain mathematical tools and concepts. It also serves as a counterexample to common misconceptions about continuous functions and differentiability.

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