A graph on more than two vertices is said to be -connected (or -vertex connected, or -point connected) if there does not exist a vertex cut of size whose removal disconnects the graph, i.e., if the vertex connectivity . Therefore, a connected graph on more than one vertex is 1-connected and a biconnected graph on more than two vertices is 2-connected.
The singleton graph is "annoyingly inconsistent" (West 2000, p. 150) since it is connected (specifically, 1-connected), but by convention it is taken to have .
The wheel graph is the "basic 3-connected graph" (Tutte 1961; Skiena 1990, p. 179).
-connectedness graph checking is implemented in the Wolfram Language as KVertexConnectedGraphQ[g, k].
The following table gives the numbers of -connected graphs for -node graphs (counting the singleton graph as 1-connected and the path graph as 2-connected).
OEIS | -connected graphs on 1, 2, ... nodes | |
1 | A001349 | 1, 1, 2, 6, 21, 112, 853, 11117, 261080, ... |
2 | A002218 | 0, 1, 1, 3, 10, 56, 468, 7123, 194066, ... |
3 | A006290 | 0, 0, 0, 1, 3, 17, 136, 2388, 80890, ... |
4 | A086216 | 0, 0, 0, 0, 1, 4, 25, 384, 14480, ... |
5 | A086217 | 0, 0, 0, 0, 0, 1, 4, 39, 1051, 102630, 22331311, ... |
6 | A324240 | 0, 0, 0, 0, 0, 0, 1, 5, 59, 3211, 830896, ... |
7 | A324092 | 0, 0, 0, 0, 0, 0, 0, 1, 5, 87, 9940, 7532629, ... |