Minimum spanning tree

Given a connected, undirected graph, a spanning tree of that graph is a subgraph which is a tree and connects all the vertices together. A single graph can have many different spanning trees. We can also assign a weight to each edge, which is a number representing how unfavorable it is, and use this to assign a weight to a spanning tree by computing the sum of the weights of the edges in that spanning tree. A minimum spanning tree or minimum weight spanning tree is then a spanning tree with weight less than or equal to the weight of every other spanning tree. More generally, any undirected graph has a minimum spanning forest.

Related Topics:
Connected - Undirected graph - Spanning tree - Subgraph - Tree

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One example would be a cable TV company laying cable to a new neighborhood. If it is constrained to bury the cable only along certain paths, then there would be a graph representing which points are connected by those paths. Some of those paths might be more expensive, because they are longer, or require the cable to be buried deeper; these paths would be represented by edges with larger weights. A spanning tree for that graph would be a subset of those paths that has no cycles but still connects to every house. There might be several spanning trees possible. A minimum spanning tree would be one with the lowest total cost.

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In case of a tie, there could be several minimum spanning trees; in particular, if all weights are the same, every spanning tree is minimum. However, one theorem states that if each edge has a distinct weight, the minimum spanning tree is unique. This is true in many realistic situations, such as the one above, where it's unlikely any two paths have exactly the same cost. This generalizes to spanning forests as well.

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