<P> When formatting is difficult, this can also be written "A ∩ A ∩ A ∩ ...". This last example, an intersection of countably many sets, is actually very common; for an example see the article on σ - algebras . </P> <P> Note that in the previous section we excluded the case where M was the empty set (∅). The reason is as follows: The intersection of the collection M is defined as the set (see set - builder notation) </P> <Dl> <Dd> ⋂ A ∈ M A = (x: ∀ A ∈ M, x ∈ A). (\ displaystyle \ bigcap _ (A \ in M) A = \ (x: \ forall A \ in M, x \ in A \).) </Dd> </Dl> <Dd> ⋂ A ∈ M A = (x: ∀ A ∈ M, x ∈ A). (\ displaystyle \ bigcap _ (A \ in M) A = \ (x: \ forall A \ in M, x \ in A \).) </Dd>

Intersection of two sets with no common elements