15-Minimum Spanning Trees

# 15-Minimum Spanning Trees - Discuss Graphs directed and...

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Discuss Graphs, directed and undirected, adjacency lists representation and adjacency matrix representation both can also be used for weighted graphs. Spanning Trees Given undirected connected graph G with no more than one edge between any two vertices and weights (real numbers) assigned to each edge - find a spanning tree whose edge weights sum up to a minimal value (no other spanning tree has a sum less than the minimal value). This spanning tree is called a minimum spanning tree (MST)) . Assume there are e edges and n vertices and the weight of an edge between vertices u and v is w(u, v). Generic Algorithm Set A to empty. While (A is not a spanning tree) Add a proper edge to A Let V be the set of vertices of G and assume the graph with vertices V and edges A can be extended to become a minimum spanning tree for G. An edge (u, v) is proper if it can be added to A and the new A can be extended to a minimum spanning tree for G. Initially, let A contain an edge selected by either; a) picking any vertex u and selecting a minimal weight edge emanating from it, or b) picking a minimal weight edge among all edges of G. Suppose the edge selected is (u, v). Let T be a minimal spanning tree for G. If T contains (u, v) then it is clearly proper. Otherwise there is a path in T from u to v and hence adding (u, v) to the edges of T creates a cycle which includes vertices u and v. Now, there must be an edge (u, y) with y not v on this path and w(u, v) cannot exceed w(u, y) because of how it was picked. Thus, removing (u, y) from A and replacing it by (u, v) gives a new minimal spanning tree for G which does contain (u, v). So, (u, v) is proper. Now, suppose we have any non-empty A that can be extended to a minimal spanning tree of G. In general, A represents a forest composed of trees. Add an edge to A by either a) selecting any one of its trees and then selecting a minimal weight edge among all edges that connect a vertex of the selected tree and any vertex not in the tree, or b) select any minimal weight edge not in A that can be added to A without creating a cycle.

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