Is there some $k$ such that if $G$ has girth $\geq k$ (i.e. $G$ contains no cycles of length $<k$) then $\chi(G)\leq 3$?
Is there some $k$ such that if $G$ has girth $\geq k$ (i.e. $G$ contains no cycles of length $<k$) then $\chi(G)\leq 3$?
Wormald [Wo79] has constructed a unit distance graph with $\chi(G)=4$ and girth $5$, with $6448$ vertices. O'Donnell [OD94] has constructed a unit distance graph with $\chi(G)=4$ and girth $4$, with $56$ vertices. Chilakamarri [Ch95] has constructed an infinite family of unit distance graphs with $\chi(G)=4$ and girth $4$, the smallest of which has $47$ vertices.