# Lecture 10: The Completeness of the Real Numbers and Basic Properties of Infinite Series | Summary and Q&A

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June 21, 2022
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Lecture 10: The Completeness of the Real Numbers and Basic Properties of Infinite Series

## TL;DR

Convergent and Cauchy sequences are both important in analysis, with convergent sequences having a limit and Cauchy sequences having elements that get close to each other.

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### Q: What is the definition of a Cauchy sequence?

A sequence is Cauchy if for any positive epsilon, there exists a natural number M such that the difference between any two terms with indices n and k greater than or equal to M is less than epsilon.

### Q: What is the difference between a convergent and a Cauchy sequence?

A convergent sequence has a limit as n approaches infinity, while a Cauchy sequence has terms that get arbitrarily close to each other.

### Q: How do you determine if a series converges?

A series converges if the sequence of partial sums converges, meaning that the sum of the terms in the series approaches a finite number as more terms are added.

### Q: What is the relationship between Cauchy sequences and convergent sequences?

A sequence is Cauchy if and only if it is convergent.

## Summary & Key Takeaways

• A sequence is Cauchy if for any positive epsilon, there exists a natural number M such that the difference between any two terms with indices n and k greater than or equal to M is less than epsilon.

• A series is convergent if the sequence of partial sums converges, meaning that the terms of the series add up to a finite number as more terms are added.