A (Slightly) Improved Approximation Algorithm for Metric TSP
Anna R. Karlin, Nathan Klein, Shayan Oveis Gharan
本文针对度量TSP问题，提出了一种略微突破3/2近似比的随机近似算法。
对于某个ε>10^-36，我们给出了一种针对度量TSP的随机3/2-ε近似算法。
For some $ε> 10^{-36}$ we give a randomized $3/2-ε$ approximation algorithm for metric TSP.
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In this paper we investigate the integrality ratio of the standard LP relaxation for the metric $s-t$ Path TSP. We make a near-optimal choice for an auxiliary function used in the analysis of Traub and Vygen which leads to an improved upper bound on the integrality ratio of 1.5273.
A (Slightly) Improved Bound on the Integrality Gap of the Subtour LP for TSP
本文证明了度量TSP中subtour LP的整数间隙至多为3/2-ε（ε>10^-36），并基于最大熵算法分析为2-edge-connected multi-subgraph问题提供了优于Christofides算法的随机近似。
We show that the max entropy algorithm can be derandomized (with respect to a particular objective function) to give a deterministic $3/2-ε$ approximation algorithm for metric TSP for some $ε> 10^{-36}$. To obtain our result, we apply the method of conditional expectation to an objective function constructed in prior work which was used to certify that the expected cost of the algorithm is at most $3/2-ε$ times the cost of an optimal solution to the subtour elimination LP. The proof in this work involves showing that the expected value of this objective function can be computed in polynomial t…
We give an improved analysis of the best-of-many Christofides algorithm with lonely edge deletion, which was proposed by Sebő and van Zuylen [2016]. This implies an improved upper bound on the integrality ratio of the standard LP relaxation for the $s$-$t$-path TSP.
We design a $1.49993$-approximation algorithm for the metric traveling salesperson problem (TSP) for instances in which an optimal solution to the subtour linear programming relaxation is half-integral. These instances received significant attention over the last decade due to a conjecture of Schalekamp, Williamson and van Zuylen stating that half-integral LP solutions have the largest integrality gap over all fractional solutions. So, if the conjecture of Schalekamp et al. holds true, our result shows that the integrality gap of the subtour polytope is bounded away from $3/2$.
We show improved approximation guarantees for the traveling salesman problem on cubic graphs, and cubic bipartite graphs. For cubic bipartite graphs with n nodes, we improve on recent results of Karp and Ravi (2014) by giving a simple "local improvement" algorithm that finds a tour of length at most 5/4 n - 2. For 2-connected cubic graphs, we show that the techniques of Moemke and Svensson (2011) can be combined with the techniques of Correa, Larre and Soto (2012), to obtain a tour of length at most (4/3-1/8754)n.
Better approximation guarantee for Asymmetric TSP
本文改进了不对称TSP的近似比，低于15，并在无权有向图和给定端点的最小成本巡回问题中获得更优比值，同时改进了自然线性规划松弛的积分比上界。
Improved Smoothed Analysis of 2-Opt for the Euclidean TSP
针对欧几里得TSP的2-opt启发式，填补了高斯扰动平滑分析在低维（d≤3）的空白，并改进了所有现有平滑复杂度界。
Improved FPT Approximation for Non-metric TSP
针对非度量旅行商问题，本文提出一种以违反三角不等式的三角形数量k为参数的FPT 2.5-近似算法，改进了此前ISAAC '22工作中的3-近似结果。
We prove new results for approximating the graphic TSP and some related problems. We obtain polynomial-time algorithms with improved approximation guarantees. For the graphic TSP itself, we improve the approximation ratio to 7/5. For a generalization, the connected-$T$-join problem, we obtain the first nontrivial approximation algorithm, with ratio 3/2. This contains the graphic $s$-$t$-path-TSP as a special case. Our improved approximation guarantee for finding a smallest 2-edge-connected spanning subgraph is 4/3. The key new ingredient of all our algorithms is a special kind of ear-decomposi…
The (1,2)-TSP is a special case of the TSP where each edge has cost either 1 or 2. In this paper we give a lower bound of $\frac{3}{2}$ for the approximation ratio of the 2-Opt algorithm for the (1,2)-TSP. Moreover, we show that the 3-Opt algorithm has an exact approximation ratio of $\frac{11}{8}$ for the (1,2)-TSP. Furthermore, we introduce the 3-Opt++-algorithm, an improved version of the 3-Opt algorithm for the (1-2)-TSP with an exact approximation ratio of $\frac{4}{3}$.
Solving large traveling salesman problem (TSP) in an efficient way is a challenging area for the researchers of computer science. This paper presents a modified version of the ant colony system (ACS) algorithm called Red-Black Ant Colony System (RB-ACS) for the solutions of TSP which is the most prominent member of the combinatorial optimization problem. RB-ACS uses the concept of ant colony system together with the parallel search of genetic algorithm for obtaining the optimal solutions quickly. In this paper, it is shown that the proposed RB-ACS algorithm yields significantly better performa…
We prove new results for approximating Graphic TSP. Specifically, we provide a polynomial-time \frac{9}{7}-approximation algorithm for cubic bipartite graphs and a (\frac{9}{7}+\frac{1}{21(k-2)})-approximation algorithm for k-regular bipartite graphs, both of which are improved approximation factors compared to previous results. Our approach involves finding a cycle cover with relatively few cycles, which we are able to do by leveraging the fact that all cycles in bipartite graphs are of even length along with our knowledge of the structure of cubic graphs.
Mömke and Svensson presented a beautiful new approach for the traveling salesman problem on a graph metric (graph-TSP), which yields a $4/3$-approximation guarantee on subcubic graphs as well as a substantial improvement over the $3/2$-approximation guarantee of Christofides' algorithm on general graphs. The crux of their approach is to compute an upper bound on the minimum cost of a circulation in a particular network, $C(G,T)$, where $G$ is the input graph and $T$ is a carefully chosen spanning tree. The cost of this circulation is directly related to the number of edges in a tour output by …
In this paper we study the special case of Graphic TSP where the underlying graph is a power law graph (PLG). We give a refined analysis of some of the current best approximation algorithms and show that an improved approximation ratio can be achieved for certain ranges of the power law exponent $β$. For the value of power law exponent $β=1.5$ we obtain an approximation ratio of $1.34$ for Graphic TSP. Moreover we study the $(1,2)$-TSP with the underlying graph of $1$-edges being a PLG. We show improved approximation ratios in the case of underlying deterministic PLGs for $β$ greater than $1.6…
Given a complete edge-weighted graph G, we present a polynomial time algorithm to compute a degree-four-bounded spanning Eulerian subgraph of 2G that has at most 1.5 times the weight of an optimal TSP solution of G. Based on this algorithm and a novel use of orientations in graphs, we obtain a (3 beta/4 + 3 beta^2/4)-approximation algorithm for TSP with beta-relaxed triangle inequality (beta-TSP), where beta >= 1. A graph G is an instance of beta-TSP, if it is a complete graph with non-negative edge weights that are restricted as follows. For each triple of vertices u,v,w in V(G), c({u,v}) <= …
Improved guarantees for the a priori TSP
本文重新研究先验旅行商问题，通过分析随机抽样子集S的概率以及主路径解与最优解的差距，给出了几乎匹配的上下界，从而将近似保证改进为随机抽样小于3.1、确定性多项式时间算法小于5.9。
Approximation Algorithms for Graphic Multi-Depot TSP and Graphic Multi-Path TSP
该研究针对图多仓库TSP和图多路径TSP两种变体，改进了近似算法，将前者的近似保证提升至完全多项式时间的3/2、亚立方实例下的4/3，将后者提升至1.9304。
An improved approximation guarantee for Prize-Collecting TSP
本文针对奖品收集旅行商问题提出新近似算法，通过精细分解LP松弛解，将近似因子从1.915改进至1.774，显著缩小了与经典TSP的可近似性差距。
Improved Approximation Algorithms for (1,2)-TSP and Max-TSP Using Path Covers in the Semi-Streaming Model
本文研究了半流模型下的TSP问题，通过提出最大路径覆盖问题的$( rac{2}{3}-ε)$近似算法，进而为(1,2)-TSP和Max-TSP分别设计了近似比$( rac{4}{3}+ε)$和$( rac{7}{12}-ε)$的改进算法。
Improved space-time tradeoff for TSP via extremal set systems
本文改进了TSP的空间时间折衷，通过构造稀疏集合系统，实现了更优的折衷，同时推翻了相关猜想，拓展到其他排列问题。
A Lower Bound for the Max Entropy Algorithm for TSP
本文针对TSP最大熵算法展开研究，证明其近似比至少为1.375，表明该算法大概率无法正面证明TSP子回路LP松弛的四分之三猜想。
An Improved Upper Bound for the Euclidean TSP Constant Using Band Crossovers
本文通过改进的带交叉策略，结合模拟和集中分析，将欧几里得TSP常数上界从0.90380改进至约0.88，并提出可能进一步提高至约0.85的替代启发式方法。
We study the structure of solutions to linear programming formulations for the traveling salesperson problem (TSP). We perform a detailed analysis of the support of the subtour elimination linear programming relaxation, which leads to algorithms that find 2-matchings with few components in polynomial time. The number of components directly leads to integrality gap upper bounds for the TSP with distances one and two, for both undirected and directed graphs. Our main results concern the subtour elimination relaxation with one additional cutting plane inequality: - For undirected instances we obt…
We present a deterministic (1+sqrt(5))/2-approximation algorithm for the s-t path TSP for an arbitrary metric. Given a symmetric metric cost on n vertices including two prespecified endpoints, the problem is to find a shortest Hamiltonian path between the two endpoints; Hoogeveen showed that the natural variant of Christofides' algorithm is a 5/3-approximation algorithm for this problem, and this asymptotically tight bound in fact has been the best approximation ratio known until now. We modify this algorithm so that it chooses the initial spanning tree based on an optimal solution to the Held…
We consider the {\em MST-interdiction} problem: given a multigraph $G = (V, E)$, edge weights $\{w_e\geq 0\}_{e \in E}$, interdiction costs $\{c_e\geq 0\}_{e \in E}$, and an interdiction budget $B\geq 0$, the goal is to remove a set $R\subseteq E$ of edges of total interdiction cost at most $B$ so as to maximize the $w$-weight of an MST of $G-R:=(V,E\setminus R)$. Our main result is a $4$-approximation algorithm for this problem. This improves upon the previous-best $14$-approximation~\cite{Zenklusen15}. Notably, our analysis is also significantly simpler and cleaner than the one in~\cite{Zenk…
Maximum Entropy is a 10/7-Approximation Algorithm for the TSP on Half-Integral Cycle Cut Instances
针对TSP的半整数环割实例，证明最大熵算法达到10/7近似比，该类实例包含Subtour LP积分间隙为4/3的案例，且算法性能不低于11/8。
In the Euclidean TSP with neighborhoods (TSPN), we are given a collection of n regions (neighborhoods) and we seek a shortest tour that visits each region. As a generalization of the classical Euclidean TSP, TSPN is also NP-hard. In this paper, we present new approximation results for the TSPN, including (1) a constant-factor approximation algorithm for the case of arbitrary connected neighborhoods having comparable diameters; and (2) a PTAS for the important special case of disjoint unit disk neighborhoods (or nearly disjoint, nearly-unit disks). Our methods also yield improved approximation …
The Asymmetric Traveling Salesperson Path Problem (ATSPP) is one where, given an asymmetric metric space $(V,d)$ with specified vertices s and t, the goal is to find an s-t path of minimum length that passes through all the vertices in V. This problem is closely related to the Asymmetric TSP (ATSP), which seeks to find a tour (instead of an $s-t$ path) visiting all the nodes: for ATSP, a $ρ$-approximation guarantee implies an $O(ρ)$-approximation for ATSPP. However, no such connection is known for the integrality gaps of the linear programming relaxations for these problems: the current-best a…
Given a traveling salesman problem (TSP) tour $H$ in graph $G$ a $k$-move is an operation which removes $k$ edges from $H$, and adds $k$ edges of $G$ so that a new tour $H'$ is formed. The popular $k$-OPT heuristics for TSP finds a local optimum by starting from an arbitrary tour $H$ and then improving it by a sequence of $k$-moves. Until 2016, the only known algorithm to find an improving $k$-move for a given tour was the naive solution in time $O(n^k)$. At ICALP'16 de Berg, Buchin, Jansen and Woeginger showed an $O(n^{\lfloor 2/3k \rfloor+1})$-time algorithm. We show an algorithm which runs …
Approximating Traveling Salesman Problems Using a Bridge Lemma
本文改进了顺序TSP和k人TSP路径的近似算法，通过桥引理的改进方法，分别获得1.878和2.214的近似比，突破了传统方法的限制。
The Metric Traveling Salesman Problem (TSP) is a classical NP-hard optimization problem. The double-tree shortcutting method for Metric TSP yields an exponentially-sized space of TSP tours, each of which approximates the optimal solution within at most a factor of 2. We consider the problem of finding among these tours the one that gives the closest approximation, i.e.\ the \emph{minimum-weight double-tree shortcutting}. Burkard et al. gave an algorithm for this problem, running in time $O(n^3+2^d n^2)$ and memory $O(2^d n^2)$, where $d$ is the maximum node degree in the rooted minimum spannin…
Approximating Prize-Collecting Variants of TSP
本文针对同时推广Prize-collecting TSP和Ordered TSP的PCOTSP问题，通过结合树采样、概率顶点选取和剪枝奇偶性修正技术，提出了2.097近似算法，并扩展至Prize-collecting Multi-Path TSP获得2.41近似比。
A Better-Than-1.6-Approximation for Prize-Collecting TSP
针对奖品收集TSP，论文基于线性规划松弛解的有根树分解、剪枝与奇偶性校正，给出1.599近似算法，并将路径版Prize-Collecting Stroll改进至1.6662。
The Travelling Salesman Problem is one the most fundamental and most studied problems in approximation algorithms. For more than 30 years, the best algorithm known for general metrics has been Christofides's algorithm with approximation factor of 3/2, even though the so-called Held-Karp LP relaxation of the problem is conjectured to have the integrality gap of only 4/3. Very recently, significant progress has been made for the important special case of graphic metrics, first by Oveis Gharan et al., and then by Momke and Svensson. In this paper, we provide an improved analysis for the approach …
TSP is a classical NP-hard combinatorial optimization problem with many practical variants. LKH is one of the state-of-the-art local search algorithms for the TSP. LKH-3 is a powerful extension of LKH that can solve many TSP variants. Both LKH and LKH-3 associate a candidate set to each city to improve the efficiency, and have two different methods, $α$-measure and POPMUSIC, to decide the candidate sets. In this work, we first propose a Variable Strategy Reinforced LKH (VSR-LKH) algorithm, which incorporates three reinforcement learning methods (Q-learning, Sarsa, Monte Carlo) with LKH, for th…
A Sharper Explicit Bound on the Subtour-LP Integrality Gap for Metric TSP
本文针对度量TSP，改进了subtour-消去LP的积分间隙上界，获得随机多项式时间(3/2-ε)近似算法，显式常数ε_*>2.78621×10^-18，优于此前结果。
We consider the rooted orienteering problem in Euclidean space: Given $n$ points $P$ in $\mathbb R^d$, a root point $s\in P$ and a budget $\mathcal B>0$, find a path that starts from $s$, has total length at most $\mathcal B$, and visits as many points of $P$ as possible. This problem is known to be NP-hard, hence we study $(1-δ)$-approximation algorithms. The previous Polynomial-Time Approximation Scheme (PTAS) for this problem, due to Chen and Har-Peled (2008), runs in time $n^{O(d\sqrt{d}/δ)}(\log n)^{(d/δ)^{O(d)}}$, and improving on this time bound was left as an open problem. Our main con…
We refine the recent breakthrough technique of Klartag and Lehec to obtain an improved polylogarithmic bound for the KLS constant.
Linear Programming based Reductions for Multiple Visit TSP and Vehicle Routing Problems
本研究提出基于线性规划的归约方法，可将多旅行商问题（mTSP）的LP类算法转换为多次访问多旅行商问题（MV-mTSP）的同近似因子算法，改进或匹配了该问题多类变体的当前最优近似因子，证明访问需求不会提升近似难度。
We study the metric $s$-$t$ path Traveling Salesman Problem (TSP). [An, Kleinberg, and Shmoys, STOC 2012] improved on the long standing $\frac{5}{3}$-approximation factor and presented an algorithm that achieves an approximation factor of $\frac{1+\sqrt{5}}{2}\approx1.61803$. Later [Sebő, IPCO 2013] further improved the approximation factor to $\frac{8}{5}$. We present a simple, self-contained analysis that unifies both results; our main contribution is a \emph{unified correction vector}. We compare two different linear programming (LP) relaxations of the $s$-$t$ path TSP, namely, the path ver…
Parameterized Approximation Algorithms for TSP on Non-Metric Graphs
本文提出改进的参数化近似算法，针对一般图中违反三角不等式的顶点数p和使图变为度量图的最小顶点数q，分别获得1.5和3的近似比。
Sublinear-Time and Semi-Streaming Algorithms for Path Cover and TSP
针对路径覆盖与旅行商问题，该研究提出了改进的亚线性时间和半流近似算法，提升了相关问题的近似比与时间效率，优化了现有算法的性能边界。
Improved Algorithms for Bounded-Degree (Subset) Traveling Salesman Problems
针对有界度旅行商路径问题，提出双准则近似算法，首次实现加性度违反，并改进了成本近似比，匹配Hoogeveen界。
Sublinear Algorithms for TSP via Path Covers
本文研究了旅行商问题(TSP)的次线性时间算法，通过改进最大路径覆盖问题的近似算法，显著提升了(1,2)-TSP和图解TSP的成本估计近似比，并证明了这些算法在信息论上时间最优且触及自然近似障碍。
We analyze two classic variants of the Traveling Salesman Problem using the toolkit of fine-grained complexity. Our first set of results is motivated by the Bitonic TSP problem: given a set of $n$ points in the plane, compute a shortest tour consisting of two monotone chains. It is a classic dynamic-programming exercise to solve this problem in $O(n^2)$ time. While the near-quadratic dependency of similar dynamic programs for Longest Common Subsequence and Discrete Frechet Distance has recently been proven to be essentially optimal under the Strong Exponential Time Hypothesis, we show that bit…
Online sorting and online TSP: randomized, stochastic, and high-dimensional
本文从随机化、随机输入和高维三个方向扩展了在线排序问题的研究，证明了实数在线排序的竞争比下界，给出了随机输入下改进的竞争比算法，并推广至高维情形与在线TSP变体。
Approximation Schemes for Subset TSP and Steiner Tree on Geometric Intersection Graphs
本文研究了几何交图上子集TSP和Steiner树问题的近似方案，通过构造特定子图实现近似解，并展示了结果对平面图的推广意义。
Improved Approximation Algorithms for the Multiple-Depot Split Delivery Vehicle Routing Problem
本文提出了一种改进的多仓库分货车辆路径问题的近似算法，解决了该问题的近似比率问题，并开发了适用于不同仓库数的多种近似算法。
We design and analyse approximation algorithms for the minimum-cost connected T-join problem: given an undirected graph G = (V;E) with nonnegative costs on the edges, and a subset of nodes T, find (if it exists) a spanning connected subgraph H of minimum cost such that every node in T has odd degree and every node not in T has even degree; H may have multiple copies of any edge of G. Two well-known special cases are the TSP (|T| = 0) and the s-t path TSP (|T| = 2). Recently, An, Kleinberg, and Shmoys [STOC 2012] improved on the long-standing 5/3-approximation guarantee for the latter problem a…
Recent papers on approximation algorithms for the traveling salesman problem (TSP) have given a new variant on the well-known Christofides' algorithm for the TSP, called the Best-of-Many Christofides' algorithm. The algorithm involves sampling a spanning tree from the solution the standard LP relaxation of the TSP, subject to the condition that each edge is sampled with probability at most its value in the LP relaxation. One then runs Christofides' algorithm on the tree by computing a minimum-cost matching on the odd-degree vertices in the tree, and shortcutting the resulting Eulerian graph to…
Local search is a widely-employed strategy for finding good solutions to Traveling Salesman Problem. We analyze the problem of determining whether the weight of a given cycle can be decreased by a popular $k$-opt move. Earlier work has shown that (i) assuming the Exponential Time Hypothesis, there is no algorithm to find an improving $k$-opt move in time $f(k)n^{o(k/\log k)}$ for any function $f$, while (ii) it is possible to improve on the brute-force running time of $O(n^k)$ and save linear factors in the exponent. Modern TSP heuristics show that very good global solutions can already be rea…
We give a new, strongly polynomial-time algorithm and improved analysis for the metric $s-t$ path TSP. It finds a tour of cost less than 1.53 times the optimum of the subtour elimination LP, while known examples show that 1.5 is a lower bound for the integrality gap. A key new idea is the deletion of some edges of Christofides' trees, which is then accompanied by novel arguments of the analysis: edge-deletion disconnects the trees, which are then partly reconnected by `parity correction'. We show that the arising `connectivity correction' can be achieved for a minor extra cost. On the one hand…
We study the load balanced capacitated vehicle routing problem (LBCVRP): the problem is to design a collection of tours for a fixed fleet of vehicles with capacity Q to distribute a supply from a single depot between a number of predefined clients, in a way that the total traveling cost is a minimum, and the vehicle loads are balanced. The unbalanced loads cause the decrease of distribution quality especially in business environments and exibility in the logistics activities. The problem being NP-hard, we propose two approximation algorithms. When the demands are equal, we present a (1-1/Q)p+3…
This paper presents an algorithm for the integer multiplicative inverse (mod $2^w$) which completes in the fewest cycles known for modern microprocessors, when using the native bit width $w$ for the modulus $2^w$. The algorithm is a modification of a method by Dumas, and for computers it slightly increases generality and efficiency. A proof is given, and the algorithm is shown to be closely related to the better known Newton's method algorithm for the inverse. Simple direct formulas, which are needed by this algorithm and by Newton's method, are reviewed and proven for the integer inverse modu…
A central problem in parameterized algorithms is to obtain algorithms with running time $f(k)\cdot n^{O(1)}$ such that $f$ is as slow growing function of the parameter $k$ as possible. In particular, a large number of basic parameterized problems admit parameterized algorithms where $f(k)$ is single-exponential, that is, $c^k$ for some constant $c$, which makes aiming for such a running time a natural goal for other problems as well. However there are still plenty of problems where the $f(k)$ appearing in the best known running time is worse than single-exponential and it remained ``slightly s…
We give a new fpt algorithm testing isomorphism of $n$-vertex graphs of tree width $k$ in time $2^{k\operatorname{polylog} (k)}\operatorname{poly} (n)$, improving the fpt algorithm due to Lokshtanov, Pilipczuk, Pilipczuk, and Saurabh (FOCS 2014), which runs in time $2^{\mathcal{O}(k^5\log k)}\operatorname{poly} (n)$. Based on an improved version of the isomorphism-invariant graph decomposition technique introduced by Lokshtanov et al., we prove restrictions on the structure of the automorphism groups of graphs of tree width $k$. Our algorithm then makes heavy use of the group theoretic techniq…
We propose improved exact and heuristic algorithms for solving the maximum weight clique problem, a well-known problem in graph theory with many applications. Our algorithms interleave successful techniques from related work with novel data reduction rules that use local graph structure to identify and remove vertices and edges while retaining the optimal solution. We evaluate our algorithms on a range of synthetic and real-world graphs, and find that they outperform the current state of the art on most inputs. Our data reductions always produce smaller reduced graphs than existing data reduct…
A cactus is a connected graph that does not contain $K_4 - e$ as a minor. Given a graph $G = (V, E)$ and integer $k \ge 0$, Cactus Vertex Deletion (also known as Diamond Hitting Set) is the problem of deciding whether $G$ has a vertex set of size at most $k$ whose removal leaves a forest of cacti. The current best deterministic parameterized algorithm for this problem was due to Bonnet et al. [WG 2016], which runs in time $26^kn^{O(1)}$, where $n$ is the number of vertices of $G$. In this paper, we design a deterministic algorithm for Cactus Vertex Deletion, which runs in time $17.64^kn^{O(1)}…
Improved matrix algorithms via the Subsampled Randomized Hadamard Transform
本文研究基于子采样随机哈达玛变换（SRHT）的低秩矩阵近似技术，在Frobenius范数和谱范数下给出了更优的误差界，并推导了SRHT的矩阵运算结果。
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