A directed odd cycle transversal of a directed graph (digraph) $D$ is avertex set $S$ that intersects every odd directed cycle of $D$. In the DirectedOdd Cycle Transversal (DOCT) problem, the input consists of a digraph $D$ andan integer $k$. The objective is to determine whether there exists a directedodd cycle transversal of $D$ of size at most $k$. In this paper, we settle the parameterized complexity of DOCT whenparameterized by the solution size $k$ by showing that DOCT does not admit analgorithm with running time $f(k)n^{O(1)}$ unless FPT = W[1]. On the positiveside, we give a factor $2$ fixed parameter tractable (FPT) approximationalgorithm for the problem. More precisely, our algorithm takes as input $D$ and$k$, runs in time $2^{O(k^2)}n^{O(1)}$, and either concludes that $D$ does nothave a directed odd cycle transversal of size at most $k$, or produces asolution of size at most $2k$. Finally, we provide evidence that there exists$epsilon > 0$ such that DOCT does not admit a factor $(1+epsilon)$FPT-approximation algorithm.
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机译:定向图(Digraph)$ D $的定向奇循环横向横向于Avertex设置$ S $与$ d $的每个奇数定向周期相交。在幽灵循环横向(Doct)问题中,输入包括一款数字$ D $ Andan Integer $ k $。目标是确定是否存在最多$ k $的D $ D $的循环周期横向横向。在本文中,我们通过解决方案尺寸$ k $的分数来解决Doct的参数化复杂性,通过显示Doct不会承认运行时间$ f(k)n ^ {o(1)} $除非fpt = w [1 ]。在阳性上,我们给出了2美元的2美元$ 2 $固定参数近似算法的问题。更精确地,我们的算法作为输入$ D $和$ k $,运行时间$ 2 ^ {o(k ^ 2)} n ^ {o(1)} $,并且得出结论,$ d $ de nothave奇怪的循环横向尺寸至多$ k $,或生产大小的朝向大多数$ 2k $。最后,我们提供证据表明存在$ epsilon> 0 $,使得Doct不承认因子$(1+ epsilon)$ FPT近似算法。
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