# What is the smallest number of disjoint spanning trees made a graph Hamiltonian

We are given a complete simple undirected weighted graph and its first arbitrary shortest spanning tree . We define the next graph and find on the second arbitrary shortest spanning tree . We continue similarly by finding on , etc. Let k be the smallest number of disjoint shortest spanning trees as defined above and let be the graph obtained as union of all disjoint trees.

**Question 1**. What is the smallest number of disjoint spanning trees creates a graph containing a Hamiltonian path.

**Question 2**. What is the smallest number of disjoint spanning trees creates a graph containing a shortest Hamiltonian path?

**Questions 3 and 4**. Replace in questions 1 and 2 a shortest spanning tree by a 1-tree. What is the smallest number of disjoint 1-trees creates a Hamiltonian graph? What is the smallest number of disjoint 1-trees creates a graph containing a shortest Hamiltonian cycle?

These questions are induced by the following paper Chrobak and Poljak. On common edges in optimal solutions to travelling salesman and other optimization problems, Discrete Applied Mathematics 20 (1988) 101-111.

## Bibliography

M. Chrobak and S. Poljak. On common edges in optimal solutions to travelling salesman and other optimization problems, Discrete Applied Mathematics 20 (1988) 101-111.

* indicates original appearance(s) of problem.

## Is this statement correct?

Unless I am mistaken, it appears that there does not exist any for which any of the above problems has a positive solution. For instance, let be the complete graph with vertex set , and define to be the edge-cut of consisting of all edges between and . Now define a weighting of by assigning each edge in weight 1 and every other edge weight 2. So, the subgraph (consisting of all edges of weight 1) is isomorphic to - and since has edge-disjoint spanning trees (by the Nash-Williams theorem, say), our procedure may well choose trees so that all of the edges in all of these graphs are in . But then will not have a Hamiltonian path since it is a subgraph of .

Of course, we may modify the edge weights here so that the procedure is forced to choose so that all of these trees have their edges in .