In words: π/2 is the infinite product of the even numbers squared divided by their two adjacent odd numbers.
Today this is formula is known as the Wallis Product. See https://proofwiki.org/wiki/Wallis%27s_Product and https://en.wikipedia.org/wiki/Wallis_product for a variety of proofs.
Wallis made many contributions to mathematics, including many leading to the invention of calculus. He is also credited for introducing the symbol ∞ for infinity. Read more about Wallis' life and mathematics here: https://en.wikipedia.org/wiki/John_Wallis.
Interestingly, one of the consequences of Euler’s solution of the Basel problem [1] is that when we plug π/2 into Euler's product formula for sin x we get the Wallis Product. Why? See
my notes on Euler's solution to the Basel problem: https://davidmeyer.github.io/qc/basel.pdf. The LaTeX source is here: https://www.overleaf.com/read/xsnvysdcfcvd. As always, questions/comments/corrections/* greatly appreciated.
There is even a quantum mechanical derivation of the Wallis Formula; see [2]
References
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[1] "Basel problem", https://en.wikipedia.org/wiki/Basel_problem
[2] "Quantum Mechanical Derivation of the Wallis Formula for π", https://arxiv.org/abs/1510.07813
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