There is a recursive definition of tree-depth in wikis:
$$ \operatorname{td}(G)=\begin{cases}1,&\text{if}|G|=1; \\\\ 1+\min\limits_{v\in V}\operatorname{td}(G-v),&\text{if } G\text{ is connected and }|G|>1; \\\\ \max\limits_i\operatorname{td}(G_i),&\text{otherwise};\end{cases} $$ Where $V$ is the set of vertices of $G$ and $G_i$ are the connected components of $G$.
Here is one simple Mathematica demonstration of the formula:
Clear["Global`*"];
(*Define the tree-depth function td according to the provided formula*)
TreeDepthOfOneCertainGraph[G_Graph] :=
Module[{vertices,
connectedComponents},(*If the graph has only one vertex*)
If[VertexCount[G] == 1, Return[1]];
(*If the graph is connected and has more than one vertex*)
If[ConnectedGraphQ[G],(*Compute tree-
depth by removing each vertex and taking the minimum of the \
resulting tree-depths*)vertices = VertexList[G];
Return[
1 + Min[Table[
TreeDepthOfOneCertainGraph[
Subgraph[G, DeleteCases[vertices, v]]], {v, vertices}]]];];
(*If the graph is not connected*)
connectedComponents = ConnectedComponents[G];
(*Compute the maximum tree-depth of the connected components*)
Return[Max[
TreeDepthOfOneCertainGraph /@ (Subgraph[G, #] & /@
connectedComponents)]];];
CycleGraphsVerticesNumberList = Range[3, 10, 1];
TreeDepthOfOneCertainGraph@*CycleGraph /@ CycleGraphsVerticesNumberList
Table[2 + Ceiling[Log[2, Ceiling[(n - 2)/2] + 1]], {n, CycleGraphsVerticesNumberList}]
{3, 3, 4, 4, 4, 4, 5, 5}
Solving the tree depth of a general graph is NP-hard, but I want to know if there are any special graphs, such as regular graphs, that can be effectively manually calculated or analyzed. For example, the tree depth of a ring with $n$ vertices (i.e. each vertex has exactly two neighbors) is $2+\lceil \log_2(\lceil \frac{n-2}{2} \rceil +1)\rceil$.
I would like to know if there are similar results (or at least estimates of order(asymptotic analysis), such as $\log_{2} n$) for a more general regular graph. Thank you!