Given a natural number $n \ge 4 $, what is the maximum number $\ell$ of subsets $A_1,A_2,\ldots, A_\ell$ of $\{1,2,\ldots,n\}$ satisfying:

  1. $A_i$ contains exactly $i$ elements for $1 \le i \le \ell$.
  2. None of the subsets $A_i$ is included in any other.

I tried small examples but I don't see the pattern.

Any ideas are welcome.

  • 1
    $\begingroup$ What results do you get for small $n$? $\endgroup$ – lulu Dec 14 '19 at 0:03
  • $\begingroup$ Unless I'm mistaken I found $ n = 4$ >> $l=2$ and $n=5$ >> $ l = 3$ and $n=6$ >> $ l = 3$ $\endgroup$ – ahmed Dec 14 '19 at 0:07
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    $\begingroup$ For $n=6$ what's wrong with the chain $(1), (2,3), (3,4,5), (2,4,5,6)$? $\endgroup$ – lulu Dec 14 '19 at 0:10
  • $\begingroup$ @chittychitty $(1)$,$(2,3)$, $(2,4,5)$ seems to be a maximal chain $\endgroup$ – ahmed Dec 14 '19 at 0:13
  • 1
    $\begingroup$ where does the question come from? $\endgroup$ – Bart Michels Dec 15 '19 at 15:32

For $n \geq 4$, the maximum is $n-2$.

Clearly, $\ell \geq n-1$ is impossible: wlog $A_1 = \{1\}$ and $A_2 = \{2, 3\}$. Because $\ell \geq n-1 \geq 3$, we have $\ell \neq 1, 2$ and hence $1 \notin A_\ell$ and wlog $2 \notin A_\ell$. So $\ell = |A_\ell| \leq n-2$.

We can obtain $\ell = n-2$ as follows: For $1 \leq k \leq n/2$, define $$A_k = \{2, 4, \ldots, 2(k-1) \} \cup \{2k-1\}$$ and $$A_{n-1-k} = \{2, 4, \ldots, 2(k-1) \} \cup \{2k+1, 2k+2, \ldots, n \}$$ (If $n$ is odd and $k = \frac{n-1}2$, both of the above choices work.) It is easy to check none of these sets includes another.

One can come up with these sets as follows: wlog $A_1 = \{1\}$.

All other $A_k$ must not contain $1$. Thus wlog $A_{n-2} = [3, n]$.

All other $A_k$ must not contain $1$, and must contain $2$ (otherwise they are a subsetof $A_{n-2}$). Thus wlog $A_2 = \{2, 3\}$.

All other $A_k$ must not contain $1$ and must contain $2$ but not $3$. Thus wlog $A_{n-3} = \{2 \} \cup [5, n]$.

All other $A_k$ must not contain $1$ nor $3$, and must contain $2$ and $4$ (otherwise they are a subset of $A_{n-3}$). Thus wlog $A_3 = \{2, 4, 5\}$.

All other $A_k$ must contain $2$ and $4$, but not $1$ nor $3$ nor $5$. Thus wlog $A_{n-4} = \{2, 4 \} \cup [7, n]$. And so on.

That is, up to permutation the choice of $A_k$ above is the only one: there are $n!$ possibilities.


Let the maximum $l$ for some $n$ be given by $l_n$.

The first thing to note is that $l_n \leq n-2$ for all $n\geq 4$ as the element in $A_1$ cannot be in $A_{l_n}$ and atmost one of the two elements in $A_2$ can be in $A_{l_n}$.

Now, we shall prove that $l_n = n-2$ for all $n\geq 4$.

We have that $l_4=2$ as we have $A_1=\{1\},A_2=\{2,3\}$ as a solution.

Also, $l_5=3$ as we have $A_1=\{1\}, A_2=\{2,3\}$ and $A_3=\{2,4,5\}$ as a solution.

For $n\geq 6$, let $l_k=k-2$ for all $k\leq n$.

Let $A_1=\{1\}$. Let $A_{n-2}=\{3,4,\cdots,n\}$. Now, let $3=1'$, $4=2'$ and so on. (Rename $i+2$ as $i'$ for $1\leq i\leq n-2$).

We can find $n-4$ subsets $A'_i, 1\leq i\leq n-4$ satisfying the given conditions for $\{1',2',3',\cdots,(n-2)'\}.$

Finally, let $A_k=\{2\}\cup A'_{k-1}$.

We have this as a solution as:

  1. $\{1\}\not\subseteq A_i$ for $i\geq 1$
  2. $A_k\not\subseteq A_{n-2}$ for $2\leq k\leq n-3$ as $2\in A_k$ but $2\not\in A_{n-2}$.
  3. $A_i\not\subseteq A_j$ for $2\leq i<j\leq n-3$ as we have defined the $A_i$'s in terms of a previous solution in such a way that no subset is contained in another. (We have used the fact that $l_{n-2}=n-4$)

As we have $l_5=3$ and $l_6=4$, $l_n = n-2$ for all $n\geq 4$.

For example, take $n=6$.

We have $A_1 = \{1\}$, $A_2=\{2\}\cup \{1'\}=\{2,3\}$, $A_3=\{2\}\cup\{2',3'\}=\{2,4,5\}$ and $A_4=\{3,4,5,6\}.$


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