Let a,b,c∈R be such that
a+b+c=a2+b2+c2=1,a3+b3+c3=1.
We say that a function f is a Palić function if f:R→R, f is continuous and satisfies
f(x)+f(y)+f(z)=f(ax+by+cz)+f(bx+cy+az)+f(cx+ay+bz)
for all x,y,z∈R.
Prove that any Palić function is infinitely many times differentiable and find all Palić functions.