Prove that if there are $vw ∈ E(G)$ such that $f(v) \neq f(w)$ then $G$ contains the graph $P_4$ as an induced subgraph Let $G = (V,E)$ be a graph with $V = \{1, \dots , n\}$. Consider the function $f : V \to V$ defined by $$f(v) = \min\{u\mid u \in V, d_G(v, u) \leq 2\}$$
Prove that if there are $vw \in E(G)$ such that $f(v) \neq f(w)$ then $G$ contains the graph $P_4$ as an induced subgraph

Suppose that $vw\in E$ is such that $f(v)\ne f(w)$. Since $x\ne y$, we may assume without loss of generality that $x<y$.
Now $d_G(v,w)=1\le 2$, so $x\le w$ and $y\le v$. Moreover, $d_G(w,w)=0\le 2$, so $y\le w$. Since $x<y$, it follows that $x\ne v$ and $x\ne w$. Suppose that $d_G(x,v)=1$; then the edges $xv$ and $vw$ are a path of length $2$ from $x$ to $w$, so $d_G(x,w)\le 2$, and therefore $y\le x<y$, which is absurd. Thus, $d_G(x,v)=2$, and there is a vertex $z$ distinct from $x$ and $v$ such that $xz,zv\in E$. To show that the vertices $x,z,v,w$ and the edges $xz,zv,vw$ are a $P_4$, you need only show that $w\notin\{x,z,v\}$. We already know that $w\ne x$ and $w\ne v$, so you really need only show that $w\ne z$. 
HINT: If $w=z$, then $d_G(x,w)\le 2$.
