If G has a unique minimum spanning tree, does that mean the edge weights in G are also unique? if yes why and if no why?

  • $\begingroup$ What did you try? Where did you get stuck? $\endgroup$ – David Richerby May 16 '19 at 16:22
  • $\begingroup$ I know that if the edge weights in G are unique, then G has a unique minimum spanning tree I'm just curious what we reverse the condition. $\endgroup$ – JKLM May 16 '19 at 16:23
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    $\begingroup$ Trees have a unique minimum spanning tree. $\endgroup$ – David Richerby May 16 '19 at 16:24
  • $\begingroup$ but that did not answer my question :) $\endgroup$ – JKLM May 16 '19 at 16:25
  • $\begingroup$ If you think about it a bit more, it does. Or, rather, it contains exactly enough information for you to answer your question. $\endgroup$ – David Richerby May 16 '19 at 16:29

If $G$ is a tree, it has a unique MST whatever its weights are. The weights could be unique, all the same, anything.


The question is: If $G$ has a unique MST, then are the edge weights necessarily distinct?

The answer is no. If $G$ is a tree with all edges having the same weight, then $G$ has a unique MST.

More generally, a minimum weight spanning tree must contain every edge that is a bridge (a bridge is an edge whose removal disconnects the graph). Consider a graph which contains two bridges, both having the same weight, and with the edges in the rest of the graph having distinct weights. Such a graph has a unique MST even though the edge weights are not all distinct because the bridges have the same weight.


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