 Vitali sets), i.e., the Lebesgue "σ"-algebra is strictly contained in the power set of formula_10.The first part of the definition states that the subset formula_6 of the real numbers is reduced to its outer measure by coverage by sets of open intervals.Each of these sets of intervals formula_12 covers formula_6 in the sense that when the intervals are combined together by union, they contain formula_6.