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An infinite sum of the form
is known as an infinite series. Such series appear in many areas of modern mathematics. Much of this topic was developed during the seventeenth century. Leonhard Euler continued this study and in the process solved many important problems. In this article we will explain Euler’s argument involving one of the most surprising series.
You are likely to have already met perhaps the most important series which is the geometric progression. Given constants and we want to sum
If we can make sense of the infinite sum – something known by Newton – which is
This was one of the first, and only, general results known during the seventeenth century. Another series then known was
Similar methods were used to find the sums
Now all these series converge. That is to say we can make sense of the infinite sum as a finite number. This is not true of a particularly famous series which is known as the harmonic series:
The following medieval proof that the harmonic series diverges was discovered and published by a French monk called Orseme around 1350 and relies on grouping the terms in the series as follows:
The harmonic series diverges
It follows that the sum can be made as large as we please by taking enough terms. In fact this series diverges quite slowly. A more accurate estimate of the speed of divergence can be made using the following more modern proof. This uses a technique known as the integral test which compares the graph of a function with the terms of the series. By integrating the function using calculus we can compare the sum of the series with the integral of the function and draw conclusions from this.
In this case we compare terms in the series with the area under the graph of the function . In particular, figure 1 shows that
Figure 1: The series 1/n above the graph of 1/(1+x)
Of course the integral on the right is easy. Solving this gives
Figure 2: The series 1/n below the graph of 1/x
The harmonic series generalisedThe harmonic series can be described as "the sum of the reciprocals of the natural numbers". Another series that presents itself as being similar is the "the sum of the squares of reciprocals of the natural numbers". That is to say, the series
Bust of Leibniz by Johann Gottfried Schmidt
"Infinite polynomial" - power seriesBefore solving this problem we look briefly at a piece of theory Euler used which allowed him to write the function in a particular way. This really could (or perhaps should) be the subject of an article in its own right. What Euler knew, as as we will see in a moment, is that can be written as an " infinite polynomial" in the following way:
Euler's solution to the Basel Problem
Euler was working on the Basel Problem at the age of 24 in 1731 by calculating a numerical approximation. This is an arduous task by hand with a series which converges as slowly as this. In 1735 he arrived at the following exact result:
This is a truly remarkable result. No one expected the value , the ratio of the circumference of a circle to the diameter, to appear in the formula for the sum. Euler starts with an th degree polynomial with the following properties:
has non-zero roots ,
Then may be written as a product in the following form:
We paraphrase Euler’s next claim as "what holds for a finite polynomial holds for an infinite polynomial". He applies this claim to the polynomial
which is an infinite polynomial with . Furthermore, as Euler knew, can be written as a series:
Multiplying by he obtained
This has zeros at for since these are the zeros of . We can now use the claim above and write as an infinite product and equate the two as
The second line pairs the positive and negative roots – the last line uses the difference of two squares to combine these. If you don’t believe this can be done you are right to question the logic here! Euler is being incredibly bold in his assertion that "what holds for a finite polynomial holds for an infinite polynomial". His use turns out to give the correct answer in this case!
Euler’s trick is to write in two different ways. He exploits this by expanding the right hand side. This infinite product will be very complicated but there will be a constant term and one can collect the term without too much effort as follows:
Now Euler equates the coefficients of to conclude that
Now Euler didn’t stop here – he expanded the product further and equated other coefficients to sum other series. In this way he obtained
In 1744 he obtained
by this method. In principle his method solves
for all natural numbers .
Extensions of the Basel problemIn a style typical of Euler, he not only solved the problem in hand but also used the method to solve a class of related problems. You will notice that his method only works for even powers. What then, about
You can find out more about some on Euler's work on infinite series (including a derivation of the last result) in his paper Remarques sur un beau rapport entre les series des puissances tant directes que reciproques.
About the author
Chris Sangwin is a member of staff in the School of Mathematics and Statistics at the University of Birmingham. He is a Research Fellow in the Learning and Teaching Support Network centre for Mathematics, Statistics, and Operational Research. His interests lie in mathematical Control Theory.