ISI MStat 2018 PSA Problem 10 | Dirichlet Function

This is a problem from ISI MStat 2018 PSA Problem 10 based on Dirichlet Function.

Dirichlet Function - ISI MStat Year 2018 PSA Question 10

Let $x$ be a real number. Then $\lim {m \rightarrow \infty}\left(\lim {n \rightarrow \infty} \cos ^{2 n}(m ! \pi x)\right)$

(A) does not exist for any x
(B) exists for all x
(C) exists if and only if x is irrational
(D) exists if and only if x is rational

Key Concepts

Limit

Sandwich Theorem

Check the Answer

Answer: is (B)

ISI MStat 2018 PSA Problem 10

Introduction to Real Analysis by Bertle Sherbert

Try with Hints

Check two cases separately one when x is rational and other is when x is irrational.

If $m!x$ is an integer, then $cos ^{2 n}(m ! \pi x) =1$

If x is rational $\frac{p}{q}$ , then, eventually, for large enough m, m! will be divisible by q , so that $m!x$ will be an integer, and we have $\lim {m \rightarrow \infty}\left(\lim {n \rightarrow \infty} \cos ^{2 n}(m ! \pi x)\right) =1$

If x is irrational, $m!x$ will never be an integer, and $|cos(m! {\pi } x)|<1$ , so that $\lim {m \rightarrow \infty}\left(\lim {n \rightarrow \infty} \cos ^{2 n}(m ! \pi x)\right) =0$ for all m>0 by Sandwich Theorem.

Similar Problems and Solutions

ISI MStat 2018 PSA Problem 10 — Outstanding Statistics Program with Applications

Outstanding Statistics Program with Applications

Related

This is a problem from ISI MStat 2018 PSA Problem 10 based on Dirichlet Function.

Dirichlet Function - ISI MStat Year 2018 PSA Question 10

Let $x$ be a real number. Then $\lim {m \rightarrow \infty}\left(\lim {n \rightarrow \infty} \cos ^{2 n}(m ! \pi x)\right)$

(A) does not exist for any x
(B) exists for all x
(C) exists if and only if x is irrational
(D) exists if and only if x is rational

Key Concepts

Limit

Sandwich Theorem

Check the Answer

Answer: is (B)

ISI MStat 2018 PSA Problem 10

Introduction to Real Analysis by Bertle Sherbert

Try with Hints

Check two cases separately one when x is rational and other is when x is irrational.

If $m!x$ is an integer, then $cos ^{2 n}(m ! \pi x) =1$

If x is rational $\frac{p}{q}$ , then, eventually, for large enough m, m! will be divisible by q , so that $m!x$ will be an integer, and we have $\lim {m \rightarrow \infty}\left(\lim {n \rightarrow \infty} \cos ^{2 n}(m ! \pi x)\right) =1$

If x is irrational, $m!x$ will never be an integer, and $|cos(m! {\pi } x)|<1$ , so that $\lim {m \rightarrow \infty}\left(\lim {n \rightarrow \infty} \cos ^{2 n}(m ! \pi x)\right) =0$ for all m>0 by Sandwich Theorem.

Similar Problems and Solutions

ISI MStat 2018 PSA Problem 10 — Outstanding Statistics Program with Applications

Outstanding Statistics Program with Applications

Related

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2 comments on “ISI MStat 2018 PSA Problem 10 | Dirichlet Function”

Arnab Chattopadhyay. says:

June 7, 2020 at 10:12 pm

What I can see that the limit exists for all real $x.$ If $x$ is rational the limit evaluates to $1$ and if $x$ is irrational the limit evaluates to $0.$ Then why do say that the limit doesn't exist for any real $x$ ? The reasoning is not quite clear to me. Can you please explain?

Reply
1. Pravat Hati says:
  
  June 7, 2020 at 11:25 pm
  
  Don't worry. It will be (B) i.e limit exists for all real x we have also shown that . I mistakenly put A as option . Between thanks for your valuable reply.
  
  Reply

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