Good Morning. I am reading the first pages of the "Lessons of Functional Analysis" of Riesz and Nagy, because I learned that this book was the main source for Apostol chapter about the Lebesgue Integral, and I am trying to find a proof of the Fundamental Theorem of Calculus for Lebesgue Integrals worked out within this approach (not Measure Theory). One of the first theorems of Riesz-Nagy's book states: "Every monotonic function f(x) posseses a finite derivative at every point x with the possible exception of the points x of a set of measure zero". To proof this T they use this lemma: "Let g(x) be a continuous function defined in the closed interval (a, b), and let E be the set of points x interior to this interval and such that there exists and e lying to the right of x with g(e) > g(x). Then the set E is either empty or an open set, i.e., it decomposes into a finite number or a denumerable infinity of open and disjoint intervals (a_k, b_k) (the underline is mine), and g(a_k) < g(b_k) for all these intervals". Well I suppose I can follow the lemma's proof on the book but in this statement there is something I did not know. Why to be E an opet set implies that it decomposes into a finite number or a denumerable infinity of open and disjoint intervals ??? Thanks for your answers.