Waste less time on Facebook — follow Brilliant.

Cool functions

Find all functions \(f:\mathbb{R} \rightarrow \mathbb{R}\) such that


for all real numbers \(x\) and \(y\).

Note by Sharky Kesa
1 year, 8 months ago

No vote yet
1 vote


Sort by:

Top Newest

Let \(P(x,y)\) be the statement \(f(f(x)+y) = x+f(f(y))\).

\(P(x,0)\) implies \(f(f(x)) = x + f(f(0))\). Applying this to \(P\) gives \(f(f(x)+y) = x+y+f(f(0))\).

\(P(x,f(0))\) implies \(f(f(x)+f(0)) = x+f(0)+f(f(0))\). \(P(0,f(x))\) implies \(f(f(0)+f(x)) = f(x)+f(f(0))\). Equating the two gives \(x+f(0)+f(f(0)) = f(x)+f(f(0))\), or \(f(x) = x + c\) for some fixed real number \(c\) for all real \(x\). It can be easily verified to satisfy the equation. Ivan Koswara · 1 year, 8 months ago

Log in to reply


Problem Loading...

Note Loading...

Set Loading...