Let S be the complement of A1∪A2∪⋯∪A11 in A; we wish to prove that ∣S∣≤60. For ℓ≥0, define
θ(ℓ)=(1−2ℓ)(1−3ℓ)=1−32ℓ+31(2ℓ).
Note that θ(0)=1 and θ(ℓ)≥0 for any integer ℓ>0. For n∈A, let ℓ(n) be the number of sets among A1,…,A11 containing n. Since S is the intersection of the complements of the Ai, we see that
∣S∣≤n∈A∑θ(ℓ(n)).
On the other hand, we have
n∈A∑θ(ℓ(n))=n∈A∑(1−32ℓ(n)+31(2ℓ(n)))=∣A∣−32i∑∣Ai∣+31i<j∑∣Ai∩Aj∣.
Putting these two equations together, we obtain
∣S∣≤225−32⋅11⋅45+31⋅(211)⋅9=60,
and therefore ∣A1∪A2∪⋯∪A11∣≥165.
It remains to give an example showing that this lower bound is best possible. Let p1,p2,…,p11 be a set of 11 distinct primes, and let A′ denote the set of all products of three of these primes. Let A′′={q1,q2,q3,…,q60} be a set of 60 distinct positive integers that are all coprime to p1,…,p11. Set A=A′∪A′′, and define
Ai={n∈A′:pi∣n}.
Then ∣Ai∣=(210)=45, ∣Ai∩Aj∣=(19)=9, and
∣A1∪A2∪⋯∪A11∣=∣A′∣=(311)=165.
Finally, ∣A∣=∣A′∣+∣A′′∣=165+60=225, so this A and A1,…,A11 give a valid example.