Mathematics | Independent Sets, Covering and Matching - GeeksforGeeks

13 Dec, 2019
1. Independent Sets –
A set of vertices I is called independent set if no two vertices in set I are adjacent to each other or in other words the set of non-adjacent vertices is called independent set.
It is also called a stable set.
The parameter α0(G) = max { |I|: I is an independent set in G } is called independence number of G i.e the maximum number of non-adjacent vertices.
Any independent set I with |I| = α0(G) is called a maximum independent set.For above given graph G, Independent sets are:I1 = {1}, I2 = {2}, I3 = {3}, I4 = {4}
I5 = {1, 3} and I6 = {2, 4} Therefore, maximum number of non-adjacent vertices i.e Independence number α0(G) = 2.
For above given graph G, Independent sets are:
I1 = {1}, I2 = {2}, I3 = {3}, I4 = {4}
I5 = {1, 3} and I6 = {2, 4} 
Therefore, maximum number of non-adjacent vertices i.e Independence number α0(G) = 2.
2. Vertex Covering –
A set of vertices K which can cover all the edges of graph G is called a vertex cover of G i.e. if every edge of G is covered by a vertex in set K.
The parameter β0(G) = min { |K|: K is a vertex cover of G } is called vertex covering number of G i.e the minimum number of vertices which can cover all the edges.
Any vertex cover K with |K| = β0(G) is called a minimum vertex cover.For above given graph G, Vertex cover is:V1 = {1, 3}, V2 = {2, 4}, 
V3 = {1, 2, 3}, V4 = {1, 2, 3, 4}, etc.  Therefore, minimum number of vertices which can cover all edges, i.e., Vertex covering number β0(G) = 2.
For above given graph G, Vertex cover is:
V1 = {1, 3}, V2 = {2, 4}, 
V3 = {1, 2, 3}, V4 = {1, 2, 3, 4}, etc.  
Therefore, minimum number of vertices which can cover all edges, i.e., Vertex covering number β0(G) = 2.
Notes –
I is an independent set in G iff V(G) – I is vertex cover of G.
For any graph G, α0(G) + β0(G) = n, where n is number of vertices in G.
Edge Covering –
A set of edges F which can cover all the vertices of graph G is called a edge cover of G i.e. if every vertex in G is incident with a edge in F.
The parameter β1(G) = min { |F|: F is an edge cover of G } is called edge covering number of G i.e sum of minimum number of edges which can cover all the vertices and number of isolated vertices(if exist).
Any edge cover F with |F| = β1(G) is called a minimum edge cover.
For above given graph G, Edge cover is:
E1 = {a, b, c, d}, 
E2 = {a, d} and E3 = {b, c}.  
Therefore, minimum number of edges which can cover all vertices, i.e., Edge covering number β1(G) = 2.
Note – For any graph G, α1(G) + β1(G) = n, where n is number of vertices in G.
3. Matching –
The set of non-adjacent edges is called matching i.e independent set of edges in G such that no two edges are adjacent in the set.
he parameter α1(G) = max { |M|: M is a matching in G } is called matching number of G i.e the maximum number of non-adjacent edges.
Any matching M with |M| = α1(G) is called a maximum matching.
For above given graph G, Matching are:
M1 = {a}, M2 = {b}, M3 = {c}, M4 = {d}
M5 = {a, d} and M6 = {b, c} 
Therefore, maximum number of non-adjacent edges i.e matching number α1(G) = 2.
Complete Matching:A matching of a graph G is complete if it contains all of G’svertices. Sometimes this is also called a perfect matching.HALL’S MARRIAGE THEOREM: The bipartite graph G =(V, E) with bipartition (V1, V2) has a complete matching from V1 to V2 if and only if |N (A)| > |A| for all subsets A of V1. (This is both necessary and sufficient condition for complete matching.)
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