On the connected detour number of a graph

For two vertices u and v in a graph \(G = (V, E)\), the detour distance \(D(u, v)\) is the length of a longest \(u–v\) path in \(G\). A \(u–v\) path of length \(D(u, v)\) is called a \(u–v\) detour. A set \(S ⊆ V\) is called a detour set of \(G\) if every vertex in \(G\) lies on a detour joining a pair of vertices of \(S\). The detour number \(dn(G)\) of G is the minimum order of its detour sets and any detour set of order \(dn(G)\) is a detour basis of \(G\). A set \(S ⊆ V\) is called a connected detour set of \(G\) if S is detour set of \(G\) and the subgraph \(G[S]\) induced by S is connected. The connected detour number \(cdn(G)\) of \(G\) is the minimum order of its connected detour sets and any connected detour set of order \(cdn(G)\) is called a connected detour basis of \(G\). Graphs G with detour diameter \(D ≤ 4\) are characterized when \(cdn(G) = p\), \(cdn(G) = p−1\), \(cdn(G) = p−2\) or \(cdn(G) = 2\). A subset \(T\) of a connected detour basis \(S\) of \(G\) is a forcing subset for \(S\) if \(S\) is the unique connected detour basis containing \(T\). The forcing connected detour number \(fcdn(S)\) of \(S\) is the minimum cardinality of a forcing subset for \(S\). The forcing connected detour number \(fcdn(G)\) of \(G\) is \(min{fcdn(S)}\), where the minimum is taken over all connected detour bases \(S\) in \(G\). The forcing connected detour numbers of certain classes of graphs are determined. It is also shown that for each pair \(a\), \(b\) of integers with \(0 ≤ a < b\) and \(b ≥ 3\), there is a connected graph \(G\) with \(fcdn(G) = a\) and \(cdn(G) = b\).