Let $f(d)$ be the maximal acyclic chromatic number of any graph with maximum degree $d$ - that is, the vertices of any graph with maximum degree $d$ can be coloured with $f(d)$ colours such that there is no edge between vertices of the same colour and no cycle containing only two colours.Estimate $f(d)$. In particular is it true that $f(d)=o(d^2)$?