The Unapologetic Mathematician

Mathematics for the interested outsider

Examples of Convergent Series

Today I want to give two examples of convergent series that turn out to be extremely useful for comparisons.

First we have the geometric series whose terms are the sequence a_n=a_0r^n for some constant ratio r. The sequence of partial sums is


If r\neq1 we can multiply this sum by \frac{1-r}{1-r} to find


Then as n goes to infinity, this sequence either blows up (for |r|>1) or converges to \frac{a_0}{1-r} (for |r|<1). In the border case r=\pm1 we can also see that the sequence of partial sums fails to converge. Thus the geometric series converges if and only if |r|<1, and we have a nice simple formula telling us the sum.

The other one I want to hit is the so-called p-series, whose terms are a_n=n^{-p} starting at n=1. Here we use the integral test to see that


so the sum and integral either converge or diverge together. If p\neq1 the integral gives \frac{n^{1-p}-1}{1-p}, which converges for p>1 and diverges for p<1.

If p=1 we get \ln(n), which diverges. In this case, though, we have a special name for the limit of the difference D. We call it “Euler’s constant”, and denote it by \gamma. That is, we can write


where e(n) is an error term whose magnitude is bounded by \frac{1}{n}.

In general we have no good value for the sums of these series, even where they converge. It takes a bit of doing to find \sum\frac{1}{n^2}=\frac{\pi^2}{6}, as Euler did in 1735 (solving the “Basel Problem” that had stood for almost a century), and now we have values for other even natural number values of p. The sum \sum\frac{1}{n^3} is known as Apéry’s constant, after Roger Apéry who showed that it was irrational in 1979. Yes, we didn’t even know whether it was a rational number or not until 30 years ago. We have basically nothing about odd integer values of p.

If we say s instead of p, and let s take complex values (no, I haven’t talked about complex numbers yet, but some of you know what they are) we get Riemann’s function \zeta(s)=\sum\frac{1}{n^s}, which is connected to some of the deepest outstanding questions in mathematics today.

April 29, 2008 - Posted by | Analysis, Calculus


  1. […] if we set , this tells us that . Then the comparison test with the geometric series tells us that […]

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  2. […] the complex norm is multiplicative, everything for the geometric series goes through again: if , and it diverges if . The case where is more complicated, but it can be […]

    Pingback by Convergence of Complex Series « The Unapologetic Mathematician | August 28, 2008 | Reply

  3. […] the final summation converges because it’s a geometric series with initial term and ratio . This implies that […]

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  4. […] this is a chunk of a geometric series; since , the series must converge, and so we can make this sum as small as we please by choosing […]

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