The **ratio test** and the **root test** are two more ways of checking for convergence of infinite series. The **ratio test** asks whether, in the limit that the number of terms goes to infinity (**n → ∞**), the ratio of the (**n+1**)^{th} term to the **n ^{th}** term is less than one.

The **root test** checks whether the limit, as **n → ∞**, of the **n ^{th}** root of the

These tests (particularly the ratio test) are valuable in the next phase of learning about infinite series, when we determine what range of a variable x allows the series to converge – its *radius of convergence* or its *interval of convergence*.

First we have to take a little digression to refine our understanding of convergence of a series.

We have dealt with alternating series, and the criteria for convergence of those are somewhat easier than for non-alternating (positive) series. Here we'll take a deeper look at the meaning of convergence. First consider the series

We've shown that this is a convergent p-series, with (p = 2) > 1. We can further claim that this series **converges absolutely** because

This is called the absolute convergence test. It's worth thinking about a counterexample to understand absolute convergence. Consider the series

Under the absolute convergence test, this series fails, because taking the absolute value of the term just gives us the divergent harmonic series with terms 1/n. Yet we know that this *alternating* series does converge. We say that it **converges conditionally**, the condition being that the terms alternate sign.

The test for conditional convergence is

That is certainly the case for the alternating harmonic series.

The last case is a divergent series like the simple harmonic series:

A series like this diverges by one of our established convergence tests (in this case the integral test works well), or its terms simply do not decrease, so it fails the divergence test.

Test for **absolute convergence**

Test for **conditional convergence**

**Divergence** – the series diverges by any one of the tests for convergence, or the divergence test.

The ratio test is a pretty straightforward test for convergence, and can distinguish between absolute and conditional convergence for series with negative terms.

We'll state it here, then do some examples. Proofs of the tests and theorems on this page will be saved for last.

Then there are three possibilities for the limit, **L**:

**Solution**: We'll set up the ratio test for this series and see how it goes. In setting up these ratios remember that **n** has to become **n+1** in the numerator:

Now generally we'll get a single fraction by recognizing that division is multiplication by the reciprocal:

Now we can use the properties of factorials and the laws of exponents to do some simplification. Recall that **2 ^{n+1} = 2^{n} ยท 2^{1}** and

which reduces to

The limit is less than one, so the series converges. Hopefully, you had a pretty good suspicion that this series was convergent from the beginning, because the factorial function grows much more rapidly than an exponential function for **n > N**, some number at or beyond the point where the two functions cross.

*which* number the series converges to, just that it does. It doesn't give us the *sum* of the series, either. We would need to find that in another way.

**Solution**: First set up the ratio test:

Now, recognizing that

and using L'Hopital's rule to evaluate the limit, we get:

The series diverges. Notice that this series wouldn't have passed the divergence test, either.

**Solution**: We begin by setting up the ratio-test expression. I'm going to forgo the absolute value signs this time (just assume they're there), and I'll just write the denominator as a reciprocal product right away:

Now expand the **(n+1) ^{2}** and rearrange to keep the roots together:

The properties of limits allow us to separate this expression into a product of limits:

The properties of limits also allow us to put the power of 1/2 on the left outside of the limit ...

... to obtain a limit of **1**, which means that the ratio test is inconclusive for this series.

This series can be shown to be convergent by direct comparison to the p-series with **p = 3/2**.

The root test is used in situations where a series term or part of it is raised to the power of the index variable. If the root test isn't fairly easy to use, you probably shouldn't use it. But when it works, it often cuts to convergence or divergence quickly.

The root test isn't a good choice if a series contains factorial terms. You'll notice some similarities between how we interpret the root test and the ratio test.

Then there are three possibilities for the limit, **L**:

**Solution**: This series is a perfect candidate for the root test because taking the n^{th} root will vastly simplify it. Here goes:

Now we can use L'Hopital's rule to find the limit:

... so the series converges.

Series raised to a power of the index variable (the variable that counts in the summation) are good candidates for the root test.

**Solution**:
Here is another great candidate for the root test because of the powers of **n**. Set up the limit like this:

Because 0 < 1, the series converges. Notice that the series with terms -3/n would not converge (-3 times a harmonic series, but that's not what's going on here).

**Solution**: First set up the limit. The **n ^{th}** root gives us 2/3.

Now the **n ^{3/n}** can be rewritten as

... so the series converges.

**xaktly.com** by Dr. Jeff Cruzan is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. © 2062, Jeff Cruzan. All text and images on this website not specifically attributed to another source were created by me and I reserve all rights as to their use. Any opinions expressed on this website are entirely mine, and do not necessarily reflect the views of any of my employers. Please feel free to send any questions or comments to jeff.cruzan@verizon.net.