Cauchy's functional equation
Cauchy's functional equation is the functional equation: f ( x + y ) = f ( x ) + f ( y ) . {\displaystyle f(x+y)=f(x)+f(y).} A function f {\displaystyle f} that solves this equation is called an additive function.
Source: Wikipedia — Cauchy's functional equation (CC BY-SA 4.0)