Olympiad problems are challenging for most of us. But this one was considered "easy" for the students taking the test! (The oldest students are in high school, but have also been 13 year old students who have taken the test and gotten a gold medal).

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Sources
https://www.imo-official.org/problems.aspx
https://www.imo-official.org/year_individual_r.aspx?year=2019&column=total&order=desc
https://artofproblemsolving.co....m/community/c6h18760
https://mathproblems123.wordpr....ess.com/2019/07/18/i
https://www.youtube.com/watch?v=QVIdKRNvxxA

Quote
http://mathforum.org/library/d....rmath/view/53922.htm

Further reading
https://en.wikipedia.org/wiki/....Cauchy%27s_functiona
https://mathoverflow.net/quest....ions/53431/does-any-
https://academia.stackexchange.....com/questions/86451

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