# Infinite Series

• John Stillwell
Chapter
Part of the Undergraduate Texts in Mathematics book series (UTM)

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As we saw in the previous chapter, many calculus problems have a solution that can be expressed as an infinite series. It is therefore useful to be able to recognize important individual series and to understand their general properties and capabilities. This is the aim of the present chapter. Starting with the infinite geometric series, already known to Euclid, we discuss the handful of examples known before the invention of calculus. These include the harmonic series $$1 + 1/2 + 1/3 + 1/4 + \cdot\cdot\cdot$$, studied by Oresme around 1350, and the stunning series for the inverse tangent, sine, and cosine, discovered by Indian mathematicians in the 15th century. The invention of calculus in the 17th century released a flood of new series, mostly of the form $$a_0 + a_1x + a_2x^2 + \cdot\cdot\cdot$$ (called power series), but also some variations, such as fractional power series. The 18th century brought new applications. De Moivre (1730) used power series to find a formula for the nth term of the Fibonacci sequence 0, 1, 1, 2, 3, 5, 8,.... Euler (1748a) introduced a generalization of the harmonic series, $$1 + 1/2^s + 1/3^s + 1/4^s + \cdot\cdot\cdot$$, and showed that, for s > 1, it equals the infinite product $$(1 - 1/{2^s})^{-1}(1 - 1/3^s)^{-1}(1 - 1/5^s)^{-1} \cdot\cdot\cdot (1 - 1/p^s)^{-1} \cdot\cdot\cdot$$ over all the prime numbers p. This discovery of Euler’s opened a new path to the secrets of the primes, exploration of which continues to this day.

## Keywords

Power Series Zeta Function Geometric Series Fibonacci Sequence Harmonic Series
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

## References

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