Denjoy–Luzin–Saks theorem should not be confused with Denjoy–Luzin theorem.
In mathematics, the Denjoy–Luzin–Saks theorem states that a function of generalized bounded variation in the restricted sense has a derivative almost everywhere, and gives further conditions of the set of values of the function where the derivative does not exist. N. N. Luzin and A. Denjoy proved a weaker form of the theorem, and later strengthened their theorem.