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TIFR 2013 Problem 11 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 program 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:True/False

Consider the sequences $x_n=\sum_{1}^{n} \frac{1}{j}$ and $y_n=\sum_{1}^{n} \frac{1}{j^2}$. Then $\left\{x_n\right\}$ is Cauchy but $\left\{y_n \right\}$ is not.

## Discussion:

We are given sequence of partial sums of a very well known type of series.  $\left\{x_n\right\}$ is a divergent sequence and $\left\{y_n\right\}$ is convergent. Also, $\mathbb{R}$ is complete. So every Cauchy sequence is convergent and any convergent sequence (as always happens in metric spaces) is Cauchy.

The true statement would be $\left\{x_n\right\}$ is not Cauchy and $\left\{y_n\right\}$ is Cauchy sequence.

## Helpdesk

• What is this topic: Real Analysis
• What are some of the associated concept: Cauchy sequence, Convergent
• Book Suggestions: Introduction to Real Analysis by R.G. Bartle, D.R. Sherbert