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Find all Z → Z functions - [UKRMO 2009 Grade 11]

Source:

January 23, 2011
functionabsolute valuealgebra proposedalgebra

Problem Statement

Find all functions f:ZZf : \mathbb Z \to \mathbb Z such that f(nm)+f(n(m+2))=2f(n(m+1))m,nZ.f (n |m|) + f (n(|m| +2)) = 2f (n(|m| +1)) \qquad \forall m,n \in \mathbb Z.
Note. x|x| denotes the absolute value of the integer x.x.