Consider a graph $G=(V,E)$ and an integer parameter $k$.
I'm interested in the round complexity, in the CONGEST model, of checking if the diameter of the graph is "much larger" or "much smaller" than $k$.
Formally, assume that we need (all vertices) to report "small" if the diameter is at most $k$ and "large" if it is larger than $f(k)$ for some function $f$ (e.g., $f(k)=100k^2$).
A closely related problem asks to approximate the diameter. In this paper, it was shown that deciding whether the diameter is at most $2\ell+1$ or larger than $3\ell+1$ requires $\widetilde \Omega(n)$ rounds for any $1\le \ell\le \text{polylog}(n)$. However, this just shows that a (small) constant factor approximation is hard.
On the algorithmic front, it is possible to compute a $3/2$-approximation in $O(\sqrt{n\log n}+D)$ rounds.
Does the round complexity depends on $n$ for any function $f(k)$? Are there functions $f$ for which this dependency is reduced, say, to logarithmic?
If we allow super-linear functions $f$, is it possible to reduce the dependency on the diameter (e.g., to $f^{-1}(D)$)? (e.g., can we get a round complexity of $O(k^2 \sqrt{n})$ for $f(k)=k^2$?)