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Bhaskar Sharma
Bhaskar Sharma

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Common graph algorithms: Min-Cut algorithm (Stoer-Wagner algorithm)


Graph theory has many algorithms that power various real-world applications. Among these algorithms, the Minimum Cut Algorithm shines as a hidden gem with its unique ability to solve problems ranging from network optimization to image segmentation. In this blog post, we will try to understand the intricacies of the Minimum Cut Algorithm and its significance.

The Algorithm

The Minimum Cut Algorithm's objective is to divide the graph into two distinct subsets of nodes while minimizing the total weight of the edges crossing the partition. Unlike shortest path algorithms, which focus on the path between specific nodes, the Minimum Cut Algorithm concentrates on disconnecting the graph in the most efficient way.

Key Concepts:

Edge Weight: Similar to other graph algorithms, the edges in the graph possess weights that represent costs, capacities, or other relevant measures.

Cut: A "cut" in a graph refers to the partitioning of the graph's nodes into two separate sets. The edges crossing this partition are part of the cut.

Minimum Cut: The minimum cut is the cut that results in the lowest possible total weight of the edges crossing the partition.

Algorithm Steps

Start with the original graph.
Initialize an empty set A to keep track of the nodes in the first partition.

Main Loop:
Repeat the following steps until only two nodes remain in the graph:

Node Selection:
Choose any node v that has not been contracted before. This node will be merged into the other partition.

Contract Node:
Merge node v with the partition represented by set A. Add the edges of node v to the cut.
Update the edge weights: For each edge (v, w) where w is in partition A, update the weight to be the sum of the weights of edges (v, w) and (w, v).
Remove node v and its incident edges from the graph.

Calculate Min-Cut Value:
After all nodes are contracted, the minimum cut value is the sum of the weights of the edges that are part of the cut.

Repeat the main loop for different choices of starting nodes.
Keep track of the minimum cut value found across all iterations.

Return Minimum Cut:
Once all iterations are completed, return the minimum cut value found. The nodes that belong to the smaller partition will constitute one side of the cut.

Applications and Use Cases

The Minimum Cut Algorithm finds its application in various domains due to its its versatility and practicality:

Network Reliability: Identifying critical connections in a network to ensure robustness and reliability.

Image Segmentation: Dividing an image into segments by cutting edges that connect different regions, aiding in computer vision tasks.

Community Detection: Identifying communities or clusters within social networks to understand relationships and interactions.

VLSI Design: Optimizing integrated circuit design by identifying the weakest connections that could impact functionality.

To use Min-cut algorithm in a graph database you can use PostgreSQL's extension Apache AGE: -
More about apache age here:
Github here:
To implement Min-cut algorithms in Apache AGE, you can use drivers given here and use AGE with programming languages such as python.:

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