This number is about functional equations, especially the most famous of them,Cauchy's equationf(x + y) = f(x) + f(y); (1)where the unknown function f : S ! S has to satisfy this equation for all elementsx; y of S. Here, S can be any set in which an addition has been de ned. A lotof solutions to functional equations proposed in problems eventually end up withan application of known results about this equation, so it seems of interest toreview these results. As usual, a choice of past problems will illustrate the variousproperties under consideration.
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