Show that there is only one positive integer k such that no graph contains exactly k...

Show that there is only one positive integer k such that no graph contains exactly k spanning trees.

Expert Solution

Show that there is only one positive integer k such that no graph contains exactly k spanning trees. I feel that k is 2. And we can show by induction that it is true for k>=3k>=3.

By graph we mean a connected undirected graph.

Look at the kk-th regular polygon, k≥3k≥3. It has kk nodes and kk edges, its spanning tree is all the edges minus one. It has exactly kk spanning trees.
Adding some edges and nodes to a graph won't reduce its number of spanning trees.
A graph has exactly one spanning tree iff it is a tree (un-cyclic), otherwise it has a cycle of length k≥3k≥3, and by the preceding, it has at least kk spanning trees.
For Better understanding see the below information:-
I assume you mean simple graphs, not true for multi-graphs.

The union of the two spanning trees contains a cycle (contains too many edges to be a tree), cycles have length greater than 22. Removing any edge from a cycle leaves a connected graph, so the union of the two spanning trees has at least 33 spanning trees, each of which is a spanning tree for the original graph.

Colby Messinger answered 2 years ago

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