Given an universum $U$ and two sets $A$ and $B$ of sets of elements from $U$. I want to find (if such a pair exists) $a \in A$ and $b \in B$: $a \cap b \equiv \emptyset$. Currently I can do it only in $O(|A| \cdot |B| \cdot |U|)$, is there way to improve this?

Say, $|U| \leq 32$. Is there a way to speed algorithm up?

If it matters, one can assume, that all elements in $A \cup B$ are unique. Another variation of the problem is $A \equiv B$ and there is a need to search $a$ and $b$ from the single set.

  • $\begingroup$ if $|U|\le 32$ then $|A|,|B|\le 2^{32}$ and therefore the algorithm would be constant-time. I suppose this is not really an interesting case for you $\endgroup$
    – nir shahar
    Commented Jul 9, 2020 at 12:47
  • $\begingroup$ Oh and what data structure do you use to keep track of $A,B,U$? $\endgroup$
    – nir shahar
    Commented Jul 9, 2020 at 12:48
  • $\begingroup$ However the number of subsets of $|U|$ is limited if $|U|$ is, and therefore also $|A|,|B|$ $\endgroup$
    – nir shahar
    Commented Jul 9, 2020 at 13:41
  • $\begingroup$ $A$ and $B$ are sets of sets. Values of $|A|$ and $|B|$ capped by $2^{|U|}$. For elements of $A$, $B$ I use bitmasks of size $|U|$. For $U$ -- does not matter. $\endgroup$ Commented Jul 9, 2020 at 13:42
  • $\begingroup$ What im saying is that if $|U|$ is capped, then also the size of $A$ and $B$. Thus the time would be constant (as its capped by a constant) $\endgroup$
    – nir shahar
    Commented Jul 9, 2020 at 13:43

1 Answer 1


Set $A = B$ and $|U| = \Theta(\log |A|)$, and you run up against the Orthogonal Vectors Conjecture trying to do better than $|A|^{2-o(1)}$.


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