We discuss convergence results for geometric series and telescoping series.

Suppose that we have a series and have to determine whether it converges or diverges. To answer this question, we define a new sequence where for all . We saw previously that
  • the series converges if and only if exists.
  • the series diverges if and only if does not exist.

The definitions above give us a way to determine whether a given series converges. In fact, to determine whether converges, we can do the following.

  • Consider the associated sequence of partial sums.
  • Try to find an explicit formula for the term . If you can find such a formula, analyze .
    • If the limit exists, converges, and if we can determine that , then .
    • If does not exist, then diverges.
  • If an explicit formula for cannot be found, further analysis is needed. We’ll expound on this in later sections.

A recursive formula for

The most straightforward way to determine whether exists is to have an explicit formula for the -th term . Note that this is not an easy task; for example, can you find a formula for for the series ? It’s not too hard to write out the first several terms in the sequence , but try to find an explicit formula for !

As it turns out, there is always a recursive formula for , and this will play an important role in later sections. Suppose that we want to consider . Let’s write out the formula for .

We can make an observation by considering in a similar way.

Now returning to our expression for , we can make an observation.

We thus have the formula

If we apply this to the series , we have and . The recursive formula reads

This does not help us analyze whether actually exists. Sometimes, however, we can find an explicit formula for , and we study two special types of series for which this is possible.

Two special types of series

Geometric series

Recall that a geometric sequence is a sequence for which the ratio of successive terms is constant. If is such a sequence, then there are constants and for which .

We thus represent this sequence by the ordered list

and we have a result that characterizes the behavior of this type of sequence, which we recall now.

We can now ask when we are able to sum the terms of a geometric sequence.

Before exploring when such a series converges, note that sometimes, some preliminary algebra is necessary to recognize a series as geometric.

We can now try to determine when adding together the terms in such a series is possible; that is, we can explore for which values of and the series converges.

From our work above, we see that the -th partial sum of the geometric series is We now have an explicit formula so we can determine for which values of the limit exists. First, note that by using the limit laws,

The existence of is thus entirely determined by whether exists, and this limit is the limit of a geometric sequence! In fact,

  • if , then existsdoes not exist .
  • if or , then existsdoes not exist .

The above formula covers every case except when , but notice that so if , and , so diverges.

When , note , so in this case,

By noting that , we can combine this observation with the above argument and write the result in a theorem.

There is a useful trick that allows us to find the sum of a convergent geometric series when the lower index does not start at .

We can easily generalize this example and doing so allows us to write down a more comprehensive theorem about geometric series.

Now, try some questions to check your understanding of the above material.

Which of the following series converge?
The initial index doesn’t matter as far as convergence is concerned, it is the “tail” of the sequence that determines convergence.
Determine if the series converges or diverges. If it converges, give the value to which it converges.

Telescoping series

A second type of series for which we can find an explicit formula for are “telescoping series”. Rather than try to give a formal definition, we think of telescoping series are infinite sums for which the required addition required to find a formula for can be done so many of the intermediate terms naturally cancel. An example will make this point more clear.

We’ve just seen an example of a telescoping series. Informally, a telescoping series is one in which the partial sums reduce to just a finite sum of terms. In the last example, the partial sum only was the sum of two nonzero terms:

Summary

Now that we have seen two special types of series for which we can find an explicit formula for the -th term in the sequence of partial sums, it helps to summarize the logic that we employed.

  • Consider the associated sequence of partial sums.
  • Try to find an explicit formula for the term . If you can find such a formula, analyze .
    • If the limit exists, converges, and if we can determine that , then .
    • If does not exist, then diverges.