Outer Lebesgue Measure Definition

[atwood]

I've seen the definition that an outer Lebesgue measure is defined as
m_n^* (A) = \inf \left\{ \sum_{k=1}^{\infty} l(I_k) \, \left| \, A \subset \bigcup_{k=1}^{\infty} I_k \right}
where I_k are n-dimensional intervals and l(I_k) is the geometric length.

It is not actually clear to me if A has to be a proper subset. That is, does
A \subset \bigcup_{k=1}^{\infty} I_k
actually mean
A \subseteq \bigcup_{k=1}^{\infty} I_k
or
A \subsetneq \bigcup_{k=1}^{\infty} I_k
?

 

[Dick]

Why do you think the distinction between those two cases is important? Show that the sum of l(I_k) can be made arbitrarily close to the sum of the lengths of intervals J_k where each I_k is a proper subset of J_k. That's a good exercise.