[Image credit: https://web.archive.org/web/20191028044928/http://www-groups.dcs.st-and.ac.uk/~history/PictDisplay/Fermat.html]
References
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[1] "Pythagorean prime", https://en.wikipedia.org/wiki/Pythagorean_prime
[2] " Fermat’s theorem on sums of two squares", https://en.wikipedia.org/w/index.php?title=Fermat%27s_theorem_on_sums_of_two_squares
[3] "Christmastime is Christmas Math Time!", https://davidmeyer.github.io/qc/christmastime_is_christmas_math_time.pdf
[4] "Square Modulo 4", https://proofwiki.org/wiki/Square_Modulo_4
[5] "Proofs of Fermat's theorem on sums of two squares", https://en.wikipedia.org/w/index.php?title=Proofs_of_Fermat%27s_theorem_on_sums_of_two_squares&oldid=989540767
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