11.2 Maximum matchings and augmenting paths

Augmenting paths are key to finding maximum matchings in bipartite graphs. These paths alternate between matched and unmatched edges, revealing opportunities to increase matching size and optimize resource allocation.

Algorithms like Hungarian and Hopcroft-Karp use augmenting paths to find maximum matchings efficiently. Their correctness is backed by Berge's theorem, while time complexity varies based on graph size and density.

Maximum Matchings and Augmenting Paths

Concept of augmenting paths

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• Bipartite graphs divide vertices into two disjoint sets with edges only connecting vertices between sets (students and courses)
• Matchings pair vertices without sharing endpoints maximizes resource allocation (job assignments)
• Augmenting paths alternate between matched and unmatched edges starting and ending with unmatched vertices
• Odd-length paths reveal opportunities to increase matching size
• Existence of augmenting paths indicates non-maximum matching allows for optimization
• Exploiting augmenting paths increases matching size improves overall pairing

Application of augmenting path algorithms

• Hungarian algorithm iteratively finds augmenting paths updates matching accordingly
• Hopcroft-Karp algorithm locates multiple shortest augmenting paths in each phase improves efficiency
• Efficient path finding utilizes specialized data structures (adjacency lists, priority queues)
• Tracking visited vertices and edges prevents redundant searches optimizes performance
• Incremental matching updates maintain partial solutions throughout execution

Correctness of augmenting path algorithms

• Berge's theorem states maximum matching has no augmenting paths provides theoretical foundation
• Proof demonstrates augmenting path existence implies non-maximality establishes algorithm correctness
• Contradiction proof shows maximum matching cannot have augmenting paths reinforces theorem
• Algorithms maintain invariants ensure consistent progress towards optimal solution
• Termination occurs when no more augmenting paths exist guarantees maximum matching

Time complexity of augmenting algorithms

• Hungarian algorithm runs in $O(|V||E|)$ time scales with graph size and density
• Hopcroft-Karp achieves $O(\sqrt{|V|}|E|)$ complexity improves performance on sparse graphs
• Graph density significantly impacts runtime denser graphs require more computation
• Initial matching quality affects convergence speed better starting points reduce iterations
• Space requirements depend on graph representation methods (adjacency matrices, edge lists)
• Additional memory needed for path finding and matching storage impacts overall efficiency