We present a new approximation algorithm for the treewidth problem which finds an upper bound on the treewidth and constructs a corresponding tree decomposition as well. Our algorithm is a faster variation of Reed's classical algorithm. For the benefit of the reader, and to be able to compare these two algorithms, we start with a detailed time analysis of Reed's algorithm. We fill in many details that have been omitted in Reed's paper. Computing tree decompositions parameterized by the treewidth $k$ is fixed parameter tractable (FPT), meaning that there are algorithms running in time $\mathcal{O}(f(k) g(n))$ where $f$ is a computable function, and $g(n)$ is polynomial in $n$, where $n$ is the number of vertices. An analysis of Reed's algorithm shows $f(k) = 2^{\mathcal{O}(k \log k)}$ and $g(n) = n \log n$ for a 5-approximation. Reed simply claims time $\mathcal{O}(n \log n)$ for bounded $k$ for his constant factor approximation algorithm, but the bound of $2^{\Omega(k \log k)} n \log n$ is well known. From a practical point of view, we notice that the time of Reed's algorithm also contains a term of $\mathcal{O}(k^2 2^{24k} n \log n)$, which for small $k$ is much worse than the asymptotically leading term of $2^{\mathcal{O}(k \log k)} n \log n$. We analyze $f(k)$ more precisely, because the purpose of this paper is to improve the running times for all reasonably small values of $k$. Our algorithm runs in $\mathcal{O}(f(k)n\log{n})$ too, but with a much smaller dependence on $k$. In our case, $f(k) = 2^{\mathcal{O}(k)}$. This algorithm is simple and fast, especially for small values of $k$. We should mention that Bodlaender et al. [2016] have an algorithm with a linear dependence on $n$, and Korhonen [2021] obtains the much better approximation ratio of 2, while the current paper achieves a better dependence on $k$.

翻译：我们为树枝问题提出了一个更差的近似算法, 它在树枝上找到一个上限, 并构建了相应的树分解值。 我们的算法是Reed经典算法的更快捷的变异。 为了读者的利益, 为了能够比较这两种算法, 我们先对 Reed 的算法进行详细的时间分析。 我们填写了 Reed 文件中忽略的许多细节。 由树枝上找到一个上限的nk$的分解参数是固定的参数( FPT ), 意思是, 在时间上运行的 $ 2, (k) 美元, 也就是 美元, 美元 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 也 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 。 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 。 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 美元, 。 。 。 。 。 。 。