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1. Approximation Algorithms for Socially Fair Clustering

   Makarychev, Yury ; Vakilian, Ali ( January 2021 , Proceedings of the Conference on Learning Theory, PMLR)

   Belkin, Mikhail ; Kpotufe, Samor (Ed.)

   We present an $e^{O(p)} (\log \ell) / (\log \log \ell)$-approximation algorithm for socially fair clustering with the $\ell_p$-objective. In this problem, we are given a set of points in a metric space. Each point belongs to one (or several) of $\ell$ groups. The goal is to find a $k$-medians, $k$-means, or, more generally, $\ell_p$-clustering that is simultaneously good for all of the groups. More precisely, we need to find a set of $k$ centers $C$ so as to minimize the maximum over all groups $j$ of $\sum_{u \text{ in group } j} d(u, C)^p$. The socially fair clustering problem was independently proposed by Abbasi, Bhaskara, and Venkatasubramanian (2021) and Ghadiri, Samadi, and Vempala (2021). Our algorithm improves and generalizes their $O(\ell)$-approximation algorithms for the problem. The natural LP relaxation for the problem has an integrality gap of $\Omega(\ell)$. In order to obtain our result, we introduce a strengthened LP relaxation and show that it has an integrality gap of $\Theta((\log \ell) / (\log \log \ell))$ for a fixed p. Additionally, we present a bicriteria approximation algorithm, which generalizes the bicriteria approximation of Abbasi et al. (2021). 

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2. A refined approximation for Euclidean k-means

   https://doi.org/10.1016/j.ipl.2022.106251  

   Grandoni, Fabrizio ; Ostrovsky, Rafail ; Rabani, Yuval ; Schulman, Leonard J. ; Venkat, Rakesh ( June 2022 , Information Processing Letters)

   In the Euclidean k-Means problem we are given a collection of n points D in an Euclidean space and a positive integer k. Our goal is to identify a collection of k points in the same space (centers) so as to minimize the sum of the squared Euclidean distances between each point in D and the closest center. This problem is known to be APX-hard and the current best approximation ratio is a primal-dual 6.357 approximation based on a standard LP for the problem [Ahmadian et al. FOCS'17, SICOMP'20]. In this note we show how a minor modification of Ahmadian et al.'s analysis leads to a slightly improved 6.12903 approximation. As a related result, we also show that the mentioned LP has integrality gap at least (16+Sqrt(5))/15 > 1.2157. . 

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3. Improved Bounds in Stochastic Matching and Optimization

   Baveja, Alok ; Chavan, Amit ; Nikiforov, Andrei ; Srinivasan, Aravind ; Xu, Pan ( January 2018 , Algorithmica)

   Real-world problems often have parameters that are uncertain during the optimization phase; stochastic optimization or stochastic programming is a key approach introduced by Beale and by Dantzig in the 1950s to address such uncertainty. Matching is a classical problem in combinatorial optimization. Modern stochastic versions of this problem model problems in kidney exchange, for instance. We improve upon the current-best approximation bound of 3.709 for stochastic matching due to Adamczyk et al. (in: Algorithms-ESA 2015, Springer, Berlin, 2015) to 3.224; we also present improvements on Bansal et al. (Algorithmica 63(4):733–762, 2012) for hypergraph matching and for relaxed versions of the problem. These results are obtained by improved analyses and/or algorithms for rounding linear-programming relaxations of these problems. 

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4. Combinatorial Algorithms for Optimal Design

   Madan, V. ; Singh, M. ; Tantipongpipat, U. ; Xie, W. ( June 2019 , Annual Conference on Learning Theory)

   In an optimal design problem, we are given a set of linear experiments v1,…,vn∈Rd and k≥d, and our goal is to select a set or a multiset S⊆[n] of size k such that Φ((∑i∈Sviv⊤i)−1) is minimized. When Φ(M)=Determinant(M)1/d, the problem is known as the D-optimal design problem, and when Φ(M)=Trace(M), it is known as the A-optimal design problem. One of the most common heuristics used in practice to solve these problems is the local search heuristic, also known as the Fedorov’s exchange method (Fedorov, 1972). This is due to its simplicity and its empirical performance (Cook and Nachtrheim, 1980; Miller and Nguyen, 1994; Atkinson et al., 2007). However, despite its wide usage no theoretical bound has been proven for this algorithm. In this paper, we bridge this gap and prove approximation guarantees for the local search algorithms for D-optimal design and A-optimal design problems. We show that the local search algorithms are asymptotically optimal when kd is large. In addition to this, we also prove similar approximation guarantees for the greedy algorithms for D-optimal design and A-optimal design problems when k/d is large. 

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5. Combinatorial Algorithms for Optimal Design

   Madan, M ; Singh, M. ; Tantipongpipat, U ; Xie, W. ( June 2019 , Conference on Learning Theory, COLT 2019)

   In an optimal design problem, we are given a set of linear experiments v1,…,vn∈Rd and k≥d, and our goal is to select a set or a multiset S⊆[n] of size k such that Φ((∑i∈Sviv⊤i)−1) is minimized. When Φ(M)=Determinant(M)1/d, the problem is known as the D-optimal design problem, and when Φ(M)=Trace(M), it is known as the A-optimal design problem. One of the most common heuristics used in practice to solve these problems is the local search heuristic, also known as the Fedorov’s exchange method (Fedorov, 1972). This is due to its simplicity and its empirical performance (Cook and Nachtrheim, 1980; Miller and Nguyen, 1994; Atkinson et al., 2007). However, despite its wide usage no theoretical bound has been proven for this algorithm. In this paper, we bridge this gap and prove approximation guarantees for the local search algorithms for D-optimal design and A-optimal design problems. We show that the local search algorithms are asymptotically optimal when kd is large. In addition to this, we also prove similar approximation guarantees for the greedy algorithms for D-optimal design and A-optimal design problems when kd is large. 

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