2.10 Let X = {x1, x2,...}, = counting measure on X , and f integrable. Then...

2.10 Let X = {x1, x2,...}, µ = counting measure on X , and f integrable. Then fdµ =

f (xi). [Hint: Suppose, first, that f ≥ 0 and let sn(x) be the simple function, which is f (x) for x = x1,...,xn, and 0 otherwise.]