A <null set> $E\subseteq\mathbb R$ has Lebesgue outer measure zero. Equivalently, for every $\epsilon>0$ it can be covered by countably many open intervals whose total length is less than $\epsilon$. This definition also applies before assuming $E$ measurable; every such set is Lebesgue measurable with measure zero.

Solution

Past exam of the options II for mathematics and philosophy course of the University of Oxford / 2013 / D4 / a / i / Solution

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