The paper presents a fast combinatorial algorithm for computing a maximum size triangle-free 2-matching in a graph. The key insights are:
The search for an augmenting path can be restricted to "amenable" paths that go through any triangle contained in the current matching and the augmenting path a limited number of times. This is in contrast to the previous complex algorithm by Hartvigsen.
The algorithm uses "gadgets" and "half-edges" to disconnect certain pairs of edges and dynamically change the gadgets during the augmentation process, rather than having fixed gadgets.
The algorithm maintains an alternating structure S that contains amenable paths starting from the unsaturated vertex s. It carefully adds and removes edges/segments from S, ensuring that S has the same alternating s-reachability as the current graph G2[S].
The algorithm identifies different types of "vulnerable" triangles in the current matching and handles them appropriately by removing certain "intractable" hinges from the graph. This ensures that any augmenting path added to the matching remains triangle-free.
The algorithm and its analysis are significantly simpler than the previous complex result by Hartvigsen, while achieving the same polynomial-time complexity.
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arxiv.org
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