Let's consider a graph $G(n)$ of $n$ vertices such as no two vertices in $G$ have the same exact neighbors (different open neighbourhoods to be more specific; I wonder if this kind of graphs have already a name). Which is the maximum number of maximal cliques of such graphs? Is that amount polynomically bounded?

Additional related question: which extra conditions must such graphs have to be planar? This is in fact a related question because planar graph does indeed have a polynomically bounded amount of maximal cliques.

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    $\begingroup$ Doesn't an $n$-clique satisfy your condition? $\endgroup$
    – Ariel
    Dec 13, 2020 at 15:31
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    $\begingroup$ @Ariel But it would have the maximum number of maximal cliquies: one $\endgroup$ Dec 13, 2020 at 15:57
  • $\begingroup$ @Ariel the utility graph doesn't satify my condition and it's a 1-clique if I'm not wrong. $\endgroup$
    – ABu
    Dec 13, 2020 at 15:57
  • $\begingroup$ @Peregring-lk Any connected graph can be converted to your graph by adding a pendant vertex on each vertex of the graph. And the number of vertices only increases twice. Therefore, you can reduce your problem to a general connected graph. $\endgroup$ Dec 13, 2020 at 16:05
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    $\begingroup$ Let $M$ be the Moon–Moser graph $K_{3,3,\cdots,3}$ with $3k$ vertices $v_{i,j}$ for $i=1,2,3$ and $j=1,2,\cdots,k$. Let $G$ be $M$ with additional vertices $u_1, u_2$ and additional edges $\{v_{1,j}, u_1\}$ and $\{v_{2,j}, u_2\}$ for all $j$. Check that all neighbourhoods are different. For $k\ge2$, the number of maximum cliques is $3^k = 3^{\frac{n-2}3}=\omega((\sqrt{n}!)^c)$ for any constant $c$, where $\sqrt{n}!$ is Yuval's answer. $\endgroup$
    – John L.
    Dec 15, 2020 at 10:13

1 Answer 1


Suppose that $n$ is a perfect square, and identify its vertex set with $\{1,\ldots,\sqrt{n}\}^2$. Connect $(a,b)$ to $(c,d)$ if $a \neq c$ and $b \neq d$. You can identify a vertex from the set of its neighbors, so the open neighborhoods are all distinct. The number of maximal cliques is $\sqrt{n}!$, which isn't polynomial.

  • $\begingroup$ So for example if $n$ is $9$, would its vertex set be the cartesian product between $\{1, 2, 3\}$ with itself right? $\endgroup$
    – ABu
    Dec 13, 2020 at 16:04
  • $\begingroup$ Right, $\sqrt{9} = 3$ and $[3] = \{1,2,3\}$. $\endgroup$ Dec 13, 2020 at 16:10
  • $\begingroup$ Amazing graphs. Assuming $n$ is a perfect square, are these graphs the ones having the maximum number of cliques while satisfying my conditions? They seem like Moon-Moser graphs with just the minimal modifications to satisfy my criteria, although the number of groups, instead of being $n/3$ of size $3$, are $\sqrt n$ of size $\sqrt n$. $\endgroup$
    – ABu
    Dec 13, 2020 at 16:54
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    $\begingroup$ They’re just the first thing that came to mind. No reason to believe that this construction is optimal, though who knows... $\endgroup$ Dec 13, 2020 at 16:55

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