A deterministic $(2 + \varepsilon)$-approximation for directed feedback vertex sets in tournaments
We nearly settle the polynomial-time approximability of the Directed Feedback Vertex Set problem in tournaments. This problem is Vertex Cover-hard, and thus cannot have a $(2 - \varepsilon)$-approximation for any $\varepsilon>0$ in polynomial time assuming the Unique Games Conjecture. In the past 28 years, several work...