=+31.9. Lebesgue's density theorem. A point x is a density point of a Borel set A if A(u, u]nA)/(u-u) -> 1 as uf x and v 1 x. From Theorems 31.2 and 31.4 deduce that almost all points of A are density points. Similarly, A(u, u]n A)/(v - u) - 0 almost everywhere on Ac.