TIFR 2015 Problem 7 Solution is a part of TIFR entrance preparation series. The Tata Institute of Fundamental Research is India’s premier institution for advanced research in Mathematics. The Institute runs a graduate programme leading to the award of Ph.D., Integrated M.Sc.-Ph.D. as well as M.Sc. degree in certain subjects.

The image is a front cover of a book named Introduction to Real Analysis by R.G. Bartle, D.R. Sherbert. This book is very useful for the preparation of TIFR Entrance.

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## Problem:

Let $f$ and (g) be two functions from ([0,1]) toÂ ([0,1]) with (f) strictly increasing. Which of the following statements is always correct?

A. If (g) is continuous, then (fog) is continuous

B. IfÂ Â (f) is continuous, then (fog) is continuous

C. If (f) and (fog) is continuous, then (g) is continuous

D. If (g) and (fog) are continuous, then (f) is continuous

## Discussion:

**A: **Let (g(x)=x) for all (xin [0,1]).

(f(x)=x) for (xin [0,frac{1}{2}]) and (f(x)=5+x) forÂ (xin (frac{1}{2},1]).

Then (fog=f) and (f) is **not continuous**.

So **A** is False.

**B:Â **Reverse (f) and (g) in** A **to show that **B** is False**.**

**C: **If (f) and (fog) are continuous then (f) is 1-1 (increasing), continuous map ([0,1]to [0,1]).

(A subset [0,1] ) be closed. Then (A) is compact. (Closed subsets of compact spaces are compact).

Therefore (f(A)) is compact. (continuous image of compact set is compact).

We have that (f(A)) is a compact subset of ([0,1]). Therefore (f(A)) is closed in ([0,1]). (compact subspace of Hausdorff space is closed).

Therefore, (f) is a closed map. So (f^{-1}) is continuous.

Hence (f^{-1}ofog=g) is continuous.

So,Â **C **is True.

**D:Â **Let (g(x)=frac{x}{4}) for all (xin [0,1]).

(f(x)=x) for (xin [0,frac{1}{2}]) and (f(x)=5+x) forÂ (xin (frac{1}{2},1]).

Then (fog(x)=f(frac{x}{4})=frac{x}{4}) for allÂ (xin [0,1]).

So (fog) is continuous but (f) is not continuous.

So, **D **is False.

## Helpdesk

**What is this topic:**Real Analysis**What are some of the associated concept:**Continuity,Closed Set, Compact Set**Book Suggestions:**Introduction to Real Analysis by R.G. Bartle, D.R. Sherbert

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