I have a set $S = \{0,1,2,3,4,5,6,7,8,9\}$. $S_i \subset S$ for $i = {1,2,3,4,5}$. Any three $S_i$ has the same union, that is $S_1 \cup S_2\cup S_3 = S_1\cup S_2\cup S_4 = ...=S_3\cup S_4\cup S_5 = A$ and no $S_i\cup S_j = A$. What is lexicoraphical least of such $S_i$s?
I have to write a Python code for this and I would appreciate any help that could direct me to find a solution. What should I know to be able to solve this problem in the shortest possible period? Also, if there's a better way to formulate this problem please do leave me comments with your suggestions. If the information is not sufficient I can clarify further.

  • $\begingroup$ How do you define the 'lexicoraphical least of such $S_i$s' when there are multiple? Do you just wish to find a solution set of which any of them is minimized? Or should the greatest of the bunch be minimal? Or? $\endgroup$
    – orlp
    Jul 26, 2020 at 14:49
  • $\begingroup$ @orlp A solution set of which any of them is minimized. For example, there would be 210 possible subsets of length 6. Let's suppose I order them and I need the first 5 that satisfy the above condition. [0,1,2] < [0,1,3] -> this is what I mean by lexicographical $\endgroup$
    – Gray_Rhino
    Jul 26, 2020 at 15:07
  • 1
    $\begingroup$ You have defined, in the comment, an order for subsets. Lexicographic order. Which order do you want for sets of subsets? Also lexicographic? To exemplify what I am asking and make my example simpler I will assume that we need $2$ instead of $5$ subsets. If $S_1<S_2<S_3<S_4$ lexicographically, and $S_1,S_4$ is a solution and $S_2,S_3$ is also a solution, which one is the solution wanted? $\endgroup$
    – plop
    Jul 26, 2020 at 16:05
  • $\begingroup$ @plop I have to find all 5 subsets satisfying the above condition. $\endgroup$
    – Gray_Rhino
    Jul 27, 2020 at 1:07

1 Answer 1


I understood the problem as that we can select any $S_i$ satisfying these conditions. Then the answer is $\{0, 1, 2, 3, 4, 5\}$. The construction is unique in a certain sense.

Wlog we can assume that $S=A$ (otherwise, just remove redundant elements from $S$). I'll use $A, B, C, D, E$ instead of $S_i$. One way to arrive at a solution is to draw a Venn diagram for $A, B, C$ and to think about it:

enter image description here

What can we say about it:

  • $A \setminus (B \cup C) \ne \emptyset$. Otherwise $B \cup C = A \cup B \cup C = S$. Similarly for $B$ and $C$.

  • For $D$ we must have $A \setminus (B \cup C) \subseteq D$. Otherwise, $B \cup C \cup D \ne B \cup C \cup A = S$. Similarly for $B$ and $C$.

  • For $D$ we must have $A \cap C \setminus B \not \subseteq D$. Otherwise (just look at the diagram), $$B \cup D = B \cup (A \setminus (B \cup C)) \cup (C \setminus (A \cup B)) \cup (A \cap C \setminus B) = S$$

  • What we said for $D$ also holds for $E$. I.e. it must contain the symmetric difference but must not contain $A \cap C \setminus B$ and similar.

  • We must have $D \cup E \cup X = S$ for $X = A,B,C$. Therefore, $D \cup E \cup (A \cap B \cap C) = S$. In particular, it means that $A \cap B \cap C \not \subseteq D \cup E$, since otherwise $D \cup E = S$.

    Also, $S \setminus (A \cap B \cap C) \subseteq D \cup E$. Therefore, forgetting about symmetric difference: $$((A \cap C) \cup (A \cap B) \cup (B \cup C)) \setminus (A \cap B \cap C) \subseteq D \cup E$$ Therefore, while $A \cap C \setminus B$ doesn't belong to either $D$ or $E$, it belongs to their union.

Note that conditions above are necessary and sufficient. Putting this together:

  • $A \setminus (B \cup C)$ must have at least one element, which also belongs to both $D$ and $E$. The same for $B, C$.
  • $A \cap C \setminus B$ must have at least two elements: one belongs to $D$, another belongs to $E$. The same for other pairwise intersections.
  • $A \cap B \cap C$ has at least one element, which doesn't belong to either $D$ and $E$.

Note that we've used all $10$ elements from $S$. All sets $A,..,E$ have $6$ elements, so we just select one of them to be $S_1$, and arrange elements in the sets so that $S_1 = \{0,..,5\}$

  • $\begingroup$ @Dimitry Thank you very much for the answer. Your answer is correct for $S_1$. Any suggestions on how I can generate the other 4 subsets? I will try to understand your logic and generalize it for (n,m) instead of (5,3). $\endgroup$
    – Gray_Rhino
    Jul 27, 2020 at 1:33
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    $\begingroup$ I don't think it'll be easy to generalize. It depends on the number of sets. Even for $(4,3)$ the construction is very different: you need $D$ to contain only symmetric differences, and therefore you can find an answer with $3$ elements. I mean, this question was definitely specifically forged with exactly these parameters. $\endgroup$
    – user114966
    Jul 27, 2020 at 1:44
  • $\begingroup$ The constraints are $1\leq n\leq 9$ and $0\leq m\leq 9$. If I can find some invariance the code will take care of different combinations. Your answer is directing me to the right path. Thanks again $\endgroup$
    – Gray_Rhino
    Jul 27, 2020 at 1:55

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