For 46 years, the best known polynomial-time approximation algorithm for the metric traveling salesman problem was the 1976 Christofides-Serdyukov algorithm, guaranteeing a tour no more than 1.5 times the optimal length. In 2021, Anna Karlin, Nathan Klein, and Shayan Oveis Gharan published the first improvement, achieving a ratio of 3/2 minus a tiny but positive constant (roughly 10^-36), a landmark theoretical result even though the improvement is far too small to matter in any practical implementation, since it establishes that 3/2 is not the best possible approximation ratio for this NP-hard problem.
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For 46 years, the best known polynomial-time approximation algorithm for the metric traveling salesman problem was the 1976 Christofides-Serdyukov algorithm, guaranteeing a tour no more than 1.5 times the optimal length. In 2021, Anna Karlin, Nathan Klein, and Shayan Oveis Gharan published the first improvement, achieving a ratio of 3/2 minus a tiny but positive constant (roughly 10^-36), a landmark theoretical result even though the improvement is far too small to matter in any practical implementation, since it establishes that 3/2 is not the best possible approximation ratio for this NP-hard problem.
The Karlin-Klein-Oveis Gharan algorithm achieves a polynomial-time approximation ratio of 3/2 minus a small positive constant (approximately 10^-36) for the metric traveling salesman problem, the first improvement over the Christofides-Serdyukov algorithm's 3/2-approximation ratio since 1976.
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Limits Registry. LR-METRIC-TSP-APPROXIMATION-RATIO. Best-known approximation ratio for the metric traveling salesman problem. 2026.