Recently, I saw an interesting problem about graphs. For any integer $p \geq 2$, we call a graph a $p$-graph if it has the property that *every* group of $p$ vertices has *exactly* $1$ common neighbor, that is, each one of these $p$ vertices has an edge to some other common vertex $x$. We consider finite undirected graphs with no loops.

The $2$-graphs are actually called Friendship graphs, and they must be triangles with $1$ point in common.

So, let us first look at $3$-graphs. Consider a $3$-graph $G$. Suppose we have vertices $a$, $b$, and $c$. By assumption, they have a common neighbor $x$, so it looks something like this.

Now $a$, $b$, and $x$ must have a common neighbor. Suppose that it's not $c$. Then, we have something like this.

Now, the subgraph containing $a$, $d$, and $x$ is complete. If they have a common neighbor, then $a$, $d$, and $x$ plus that common neighbor is complete, so we have a complete graph with $4$ vertices. Call their common neighbor $v$. If $v \neq b$, we have this situation. But now, $a$, $b$, and $v$ have two common neighbors, $x$ and $d$, so in fact, $v = b$, so the complete graph is formed from $a$, $b$, $d$, and $x$.By symmetry, we'll have that $a$ and $c$ are part of a complete graph, too, which gives us this.

But now, $b$, $c$, and $d$ have $2$ common neighbors, $a$ and $x$, which is a contradiction. Thus, $d = c$ in the first place, so we actually have this.

Since this graph is isomorphic to the $a$, $b$, $d$, $v$, and $x$ subgraph earlier, we have that $a$, $c$, and $x$ have common neighbor $b$, so the only possible $3$-graph is the complete graph with $4$ vertices.

Now, the the rest of cases will follow by induction. For our induction hypothesis, assume that for $p = 3,\ldots, n - 1$, the only possible $p$-graph is the complete graph with $p + 1$ vertices. We have just proved the base case $p=3$.

Now, consider a $n$-graph for some $n > 3$. Call this graph $G$. Consider $n$ vertices, and let us call them $v_1, v_2, \ldots, v_n$. They have common neighbor $x$. Consider the subgraph consisting of only friends of $x$. Note that this graph excludes $x$. We'll call it $G_x$. Now, $G_x$ is a $n-1$-graph. To see this, note that any set of $n-1$ vertices of $G_x$ plus $x$ itself will have exactly $1$ common neighbor. Call it $y$. Since $y$ is neighbor of $x$, we must have that $y \in G_x$. Thus, $G_x$ is a $n-1$-graph, and by our induction hypothesis, $G_x$ is a complete graph with $n$ vertices. Since every vertex in $G_x$ is connected to $x$, then $G_x \cup \{x\}$ is a complete graph with $n+1$ verties.

Now, we show that $G = G_x \cup \{x\}$. Suppose otherwise for a contradiction, so there is a $y \in G$ such that $y \not\in G_x \cup \{x\}$. $y$ and $x$ have a common neighbor, so $y$ is connected with some $v_i$. But $v_i \in G_x \cup \{x\}$, which is a complete $n+1$ graph, so $v_i$ is the common neighbor of $x$ and $v_1,\ldots,v_{i-1},v_{i+1},\ldots, v_n$. We can consider $G_{v_i}$, the subgraph consisting of only friends of $v_i$. For the same reason that $G_x$ is a complete graph with $n$ vertices, $G_{v_i}$ is also a complete graph with $n$-vertices. But we have that $x,y,v_1,\ldots,v_{i-1},v_{i+1},\ldots,v_n \in G_{v_i}$, which is $n+1$ vertices, so we have a contradiction. Thus, we have that $G = G_x \cup \{x\}$. So, $G$ is a complete graph with $n+1$ vertices. This proves our induction step.

All in all, we have that for $p \geq 3$, the only possible $p$ graph is the complete graph with $p+1$ vertices.

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