Fermat's Two Squares Theorem/Historical Note

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Historical Note on Fermat's Two Squares Theorem

According to Ivan M. Niven, on "Albert Girard" at Absolute Astronomy.com, Fermat's Two Squares Theorem was initially stated without proof by Albert Girard in $1632$.

Fermat announced its proof in a letter to Marin Mersenne dated December $25$, $1640$.

For this reason it is also sometimes referred to as Fermat's Christmas Theorem.

The first published proof was by Leonhard Paul Euler after $7$ years of hard work.