Suppose that f(x) satisfies the functional equation
f(x + y) = f(x) + f(y)
for all x,y in R (the real numbers). Prove that if f(x) is continuous that f(x) = cx where c is a constant. What can you say about f(x) if it is allowed to be discontinuous?
The notion of discontinuity in relation to a given function is investigated.