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.com/community/c6h1876068
https://mathproblems123.wordpress.com/2019/07/18/imo-2019-problem-1/
https://www.youtube.com/watch?v=QVIdKRNvxxA
Quote
http://mathforum.org/library/drmath/view/53922.html
Further reading
https://en.wikipedia.org/wiki/Cauchy%27s_functional_equation
https://mathoverflow.net/questions/53431/does-any-research-mathematics-involve-solving-functional-equations
https://academia.stackexchange.com/questions/86451/does-the-international-math-olympiad-help-research-mathematics
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