Proof about concave decreasing function with some constraints Posted by Erik Satie, at math.stackexchange.com, 14 Jun 2022 Hi it's a follow up of How to prove that the functional equation $f(x)+f(y)=f(\frac{xf'(x)+yf'(y)}{f'(x)+f'(y)})+f(\frac{x+y}2)…