Spanning Tree - Category - pgRouting Manual (3.7)
Spanning Tree - Category
A spanning tree of an undirected graph is a tree that includes all the vertices of G with the minimum possible number of edges.
For a disconnected graph, there there is no single tree, but a spanning forest, consisting of a spanning tree of each connected component.
Characteristics:
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It’s implementation is only on undirected graph.
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Process is done only on edges with positive costs.
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When the graph is connected
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The resulting edges make up a tree
 
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When the graph is not connected,
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Finds a minimum spanning tree for each connected component.
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The resulting edges make up a forest.
 
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See Also
Indices and tables