Cauchy's functional equation ($f(x+y)=f(x)+f(y)$)

Cauchy's functional equation is a well-known functional equation given by: \[ f(x + y) = f(x) + f(y) \] for all real numbers \(x\) and \(y\). This equation describes a function \(f\) that satisfies the property that the value of the function at the sum of two arguments is equal to the sum of the values of the function at each argument.

Nice result on Lebesgue measurable required for uniqueness.