Computer scientists spent forty-four years convinced that a 1976 algorithm was the absolute mathematical limit for finding the shortest delivery route; Anna Karlin, Nathan Klein, and Shayan Oveis Gharan broke the 3/2 barrier using high-dimensional geometry and random spanning trees. Awarded the Best Paper at STOC, this historic breakthrough shattered the most famous stagnation in theoretical computer science.

The Traveling Salesperson Problem—finding the shortest possible route that visits a list of cities and returns home—is the holy grail of optimization. Because finding the perfect route for thousands of cities takes computers billions of years, engineers rely on fast approximation shortcuts.
In 1976, mathematician Nicos Christofides created an algorithm guaranteed to find a route within fifty percent of the optimal path. For forty-four years, thousands of mathematicians tried and failed to beat that 1.5 ratio, believing it was an unbreakable mathematical wall.
By sampling random spanning trees from higher-dimensional geometry, the University of Washington team proved that the 1.5 barrier could be broken. By reviving modern combinatorial optimization, by inspiring new algorithms for logistics delivery networks, and by proving mathematical patience wins, breaking the TSP barrier made history.
A (Slightly) Improved Approximation Algorithm for Metric TSP
In “An Improved Approximation Algorithm for TSP,” Karlin, Klein, and Oveis Gharan design the first improvement over the classical 1.5 approximation algorithm of Christofides-Serdyukov after more than 40 years. Their algorithm first chooses a random spanning tree from the maximum entropy distribution of spanning trees with marginals equal to the optimum LP solution of TSP, and then, similar to Christofides’ algorithm, it adds the minimum cost matching on the odd degree vertices of the tree. To analyze their simple algorithms, they prove and exploit new tools from the theory of strongly Rayleigh distributions.
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