# Convergence Tests

Statement

Comment

## Geometric Series and related tests.

Statement

Comment
This is the granddaddy of many series which are easy to sum. It also is the foundation for several other tests when you observe:
• The ratio of successive terms of the Geometric Series is x -- hence the Ratio Test
• The ratio of the absolute values of successive terms of the Geometric Series is |x| -- hence the Ratio Test for Absolute Convergence
• The nth root of the nth term of the Geometric Series is x -- hence the Root Test

### Ratio Test

Statement

be a series with positive terms and

then:

Comment
Try this test when uk involves factorials or kth powers.

### Ratio Test for Absolute Convergence

Statement

be a series with non-zero terms and

then:

Comment
The series need not have positive terms and need not be alternating to use this test since any series converges if it converges absolutely.

### Root Test

Statement

be a series with positive terms and

then:

Comment: Try this test when uk involves kth powers.

## Integral Test

Statement

be a series with positive terms and let f(x) be the function that results when k is replaced by x in the formula for uk.
If f is a decreasing, continuous function for x > N then:

have like convergence (either both converge or both diverge).

Comment
Use this test when f(x) is easy to integrate.

## Limit Comparison Test

Statement
be series with positive terms such that:

then the two series have like convergence (either both converge or both diverge).
then notice which series "won".
• Your unknown series converges if it is clearly smaller than a convergent series -- think about it.
• Your unknown series diverges if it is clearly larger than a divergent series -- think about it.

Comment
This is easier to apply than the Comparison Test, but still requires some skill in choosing the known series. The Divergence Test can be a great source of inspiration here.

## Comparison Test

Statement
be series with positive terms such that:

converges then converges.
Similarly, diverges, then diverges.

Comment
Use this test as a last resort. While this test is the foundation of most other tests, other tests are often easier to apply.

## Alternating Series Test

Statement

and
or equivalently
converges, provided
and
Comment
This test applies to alternating series only.

## Telescoping Series

Statement
Any series where massive cancellation of terms occurs. Often partial sums simplify to a sum of some early terms and some ending terms: everything in between sums to zero (cancels).
Comment
Any time you see individual terms involving funky arithmetic with the indices, be on the lookout for a telescoping series.
• Break up the typical term into a sum wherever possible.
• Write out the first few terms.
• Watch for developing patterns which will allow terms to cancel.

-- DickFurnas - 16 Nov 2008

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Topic revision: r4 - 2017-10-18 - SteveGaarder

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