IMO Shortlist 1999 problem A5
Dodao/la:
arhiva2. travnja 2012. Find all the functions

such that
for all

.
%V0
Find all the functions $f: \mathbb{R} \mapsto \mathbb{R}$ such that
$$f(x-f(y))=f(f(y))+xf(y)+f(x)-1$$
for all $x,y \in \mathbb{R}$.
Izvor: Međunarodna matematička olimpijada, shortlist 1999