Functions satisfying $f(x^2) = f(x)$
104369
If $f$ continuous and $f(x^2) = f(x)$, then $f$ is a const
39090
Finding functions such that $f(g(x)) = f(x)g(x)$
29647
Do there exist functions satisfying $f(x+y)=f(x)+f(y)$ that aren't linear?
122329
Determine all functions $f(x)$ such that $f(f(x+y))=f(x)+f(y)$
1071
Determine all functions $f:\mathbb{N}\rightarrow\mathbb{N}$ such that $xf(y)+yf(x)=(x+y)f(x^2+y^2)$ for all $x,y\in\mathbb{N}$ (contest question)
409
functions satisfying $f(x - y) = f(x) f(y) - f(a - x) f(a + y)$ and $f(0)=1$
55175
Is the function $f(x) = x^2 - 2x$ one-to-one?
28620
Function $f(z)$ such that $|f(z)| > |z|$ for every $z\in\mathbb{C}, 1/f(z)$
105319
A calculus problem with functions such that $f''(x) = g(x)$ and $g''(x) = f(x)$
50552
find functions that $f(m+k)=f(mk-n)$
24862
Solving (quadratic) equations of iterated functions, such as $f(f(x))=f(x)+x$
86128
Find all functions $ f : \Bbb R \to \Bbb R$ which satisfy $f(x)f(y) = f(x + y) + xy$ for all $x, y ∈ \Bbb R$.
89257
Give an example of a function $f:\mathbb{R}^2 \mapsto \mathbb{R}^2$ such that
84143
Given any function $g(x): \mathbb{R} \rightarrow \mathbb{R}$, find a function $f(x)$ such that $f(f(x)) = g(x)$.
104278
Finding function $f(x)$ which satisfy given functional equation
17154
Does a non-trivial solution exist for $f'(x)=f(f(x))$?
23186
$f: \mathbb{R^2} \to \mathbb{R}$ a $C^\infty$ function such that $f(x,0)=f(0,y)=0$ then exists $g$ such that $f(x,y)=xy\, g(x,y)$