6th July

On this day in 1708, de Moivre wrote to Johann Bernoulli about Machin's series:

(\frac{16}{5} - \frac{4}{239}) - \frac{1}{3} (\frac{16}{5^3} - \frac{4}{239^3}) + \frac{1}{5} (\frac{16}{5^5} - \frac{4}{239^5})\, ... = 3.14159 ... = \pi

, on this occasion giving two different proofs that it converges to π.
This series follows from the identity

\frac {\pi }{4}=4\tan^{-1} {\frac {1}{5}}-\tan^{-1} {\frac {1}{239}}

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The postage stamp of one of today's mathematicians at THIS LINK was issued in 2017.