# Where can i find literature about the $\frac{4}{3}$-conjecture for approximation of the Metric TSP?

In Graph-Theory there are many ways for efficient approximation-algorithms to solve the Metric TSP. The best solution seems to be the Christofides Heuristic with a factor of 1.5 to the optimal solution. My Teacher said, there would be the so called $\frac{4}{3}$-conjecture, which states: there might be a approximation solution for the metric tsp, that has only a $\frac{4}{3}$-factor.