On the robustness of functional equations

Given a functional equation, such as ∀x, y f(x)+f(y)=f(x+y), we study the following general question: When can the “for all” quantifiers be replaced by “for most” quantifiers without essentially changing the functions that are characterized by the property? When “for most” quantifiers are sufficient, we say that the functional equation is robust. We show conditions on functional equations of the form ∀x, y F[f(x-y), f(x+y), f(x), f(y)]=0, where F is an algebraic function, that imply robustness. We then initiate a general study aimed at characterizing properties of functional equations that determine whether or not they are robust. Our results have applications to the area of self-testing/correcting programs-this paper provides results which show that the concept of self-testing/correcting has much broader applications than we previously understood. We show that self-testers and self-correctors can be found for many functions satisfying robust functional equations, including tan x, 1/1+cot x, Ax/1-Ax´, cosh x