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Computing pi with tools from Calculus

Computing $\pi$

This note was originally written in the context of my fall Math 100 class at Brown University. It is also available as a pdf note.

While investigating Taylor series, we proved that
\begin{equation}\label{eq:base}
\frac{\pi}{4} = 1 – \frac{1}{3} + \frac{1}{5} – \frac{1}{7} + \frac{1}{9} + \cdots
\end{equation}
Let’s remind ourselves how. Begin with the geometric series
\begin{equation}
\frac{1}{1 + x^2} = 1 – x^2 + x^4 – x^6 + x^8 + \cdots = \sum_{n = 0}^\infty (-1)^n x^{2n}. \notag
\end{equation}
(We showed that this has interval of convergence $\lvert x \rvert < 1$). Integrating this geometric series yields
\begin{equation}
\int_0^x \frac{1}{1 + t^2} dt = x – \frac{x^3}{3} + \frac{x^5}{5} – \frac{x^7}{7} + \cdots = \sum_{n = 0}^\infty (-1)^n \frac{x^{2n+1}}{2n+1}. \notag
\end{equation}
Note that this has interval of convergence $-1 < x \leq 1$.

We also recognize this integral as
\begin{equation}
\int_0^x \frac{1}{1 + t^2} dt = \text{arctan}(x), \notag
\end{equation}
one of the common integrals arising from trigonometric substitution. Putting these together, we find that
\begin{equation}
\text{arctan}(x) = x – \frac{x^3}{3} + \frac{x^5}{5} – \frac{x^7}{7} + \cdots = \sum_{n = 0}^\infty (-1)^n \frac{x^{2n+1}}{2n+1}. \notag
\end{equation}
As $x = 1$ is within the interval of convergence, we can substitute $x = 1$ into the series to find the representation
\begin{equation}
\text{arctan}(1) = 1 – \frac{1}{3} + \frac{1}{5} – \frac{1}{7} + \cdots = \sum_{n = 0}^\infty (-1)^n \frac{1}{2n+1}. \notag
\end{equation}
Since $\text{arctan}(1) = \frac{\pi}{4}$, this gives the representation for $\pi/4$ given in \eqref{eq:base}.

However, since $x=1$ was at the very edge of the interval of convergence, this series converges very, very slowly. For instance, using the first $50$ terms gives the approximation
\begin{equation}
\pi \approx 3.121594652591011. \notag
\end{equation}
The expansion of $\pi$ is actually
\begin{equation}
\pi = 3.141592653589793238462\ldots \notag
\end{equation}
So the first $50$ terms of \eqref{eq:base} gives two digits of accuracy. That’s not very good.

I think it is very natural to ask: can we do better? This series converges slowly — can we find one that converges more quickly?

(more…)

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