Question 26
(YUG 4) Prove that the functional equations
$$f(x+y)=f(x)+f(y)$$
and
$$f(x+y+xy)=f(x)+f(y)+f(xy)\qquad(x,y\in\mathbb R)$$
are equivalent.
$$f(x+y)=f(x)+f(y)$$
and
$$f(x+y+xy)=f(x)+f(y)+f(xy)\qquad(x,y\in\mathbb R)$$
are equivalent.
IMO 1979 - Shortlisted problems Question 26
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