Ashtiani et al. (NIPS 2016) introduced a semi-supervised framework for clustering (SSAC) where a learner is allowed to make samecluster queries. More specifically, in their model, there is a query oracle that answers queries of the form "given any two vertices, do they belong to the same optimal cluster?". In many clustering contexts, this kind of oracle queries are feasible. Ashtiani et al. showed the usefulness of such a query framework by giving a polynomial time algorithm for the k-means clustering problem where the input dataset satisfies some separation condition. Ailon et al. extended the above work to the approximation setting by giving an efficient (1+ε)-approximation algorithm for k-means for any small ε > 0 and any dataset within the SSAC framework. In this work, we extend this line of study to the correlation clustering problem. Correlation clustering is a graph clustering problem where pairwise similarity (or dissimilarity) information is given for every pair of vertices and the objective is to partition the vertices into clusters that minimise the disagreement (or maximises agreement) with the pairwise information given as input. These problems are popularly known as MinDisAgree and MaxAgree problems, and MinDisAgree[k] and MaxAgree [k] are versions of these problems where the number of optimal clusters is at most k. There exist Polynomial Time Approximation Schemes (PTAS) for MinDisAgree [k] and MaxAgree [k] where the approximation guarantee is (1 + ε) for any small ε and the running time is polynomial in the input parameters but exponential in k and 1/ε. We get a significant running time improvement within the SSAC framework at the cost of making a small number of same-cluster queries. We obtain an (1+ε)-approximation algorithm for any small ε with running time that is polynomial in the input parameters and also in k and 1/ε. We also give non-trivial upper and lower bounds on the number of same-cluster queries, the lower bound being based on the Exponential Time Hypothesis (ETH). Note that the existence of an efficient algorithm for MinDisAgree[k] in the SSAC setting exhibits the power of same-cluster queries since such polynomial time algorithm (polynomial even in k and 1/ε) is not possible in the classical ⋆ Nir Ailon acknowledges the generous support of ISF grant number 2021408 ⋆⋆ Anup Bhattacharya acknowledges the support of TCS fellowship at IIT Delhi. ⋆ ⋆ ⋆ Ragesh Jaiswal acknowledges the support of ISF-UGC India-Israel Grant 2014. † Email address: nailon@cs.technion.ac.il ‡ Email addresses: {anupb, rjaiswal}@cse.iitd.ac.in (non-query) setting due to our conditional lower bounds. Our conditional lower bound is particularly interesting as it not only establishes a lower bound on the number of same cluster queries in the SSAC framework but also establishes a conditional lower bound on the running time of any (1 + ε)-approximation algorithm for MinDisAgree [k].
Abstract. The classical center based clustering problems such as k-means/median/center assume that the optimal clusters satisfy the locality property that the points in the same cluster are close to each other. A number of clustering problems arise in machine learning where the optimal clusters do not follow such a locality property. For instance, consider the r-gather clustering problem where there is an additional constraint that each of the clusters should have at least r points or the capacitated clustering problem where there is an upper bound on the cluster sizes. Consider a variant of the k-means problem that may be regarded as a general version of such problems. Here, the optimal clusters O1, ..., O k are an arbitrary partition of the dataset and the goal is to output k-centers c1, ..., c k such that the objective functionIt is not difficult to argue that any algorithm (without knowing the optimal clusters) that outputs a single set of k centers, will not behave well as far as optimizing the above objective function is concerned. However, this does not rule out the existence of algorithms that output a list of such k centers such that at least one of these k centers behaves well. Given an error parameter ε > 0, let ℓ denote the size of the smallest list of k-centers such that at least one of the k-centers gives a (1 + ε) approximation w.r.t. the objective function above. In this paper, we show an upper bound on ℓ by giving a randomized algorithm that outputs a list of 2Õ (k/ε) k-centers 1 . We also give a closely matching lower bound of 2Ω. This is a significant improvement over the previous result of Ding and Xu [DX15] who gave an algorithm with running time O nd · (log n)and output a list of size O (log n) k · 2 poly(k/ε) . Our techniques generalize for the k-median problem and for many other settings where non-Euclidean distance measures are involved.
In this study, a novel multi‐band metamaterial absorber has been proposed which is ultra‐thin, compact, polarisation‐insensitive and wide‐angle absorptive. The proposed structure comprises two concentric metallic rings printed on a dielectric substrate which is backed by a metal ground plane. The proposed structure exhibits four distinct absorption peaks at 4.11, 7.91, 10.13 and 11.51 GHz with peak absorptivities of 98.81, 99.68, 99.98 and 99.34%, respectively, under normal incidence. The designed absorber is polarisation insensitive due to four‐fold symmetry, which has been confirmed by simulation and measurement studies. Moreover, the structure shows high absorption (over 90%) for oblique incident angles up to 45° for both transverse‐electric and transverse‐magnetic polarisations. The surface current distributions at the four absorption frequencies have been illustrated to explain the absorption mechanism of the structure. In addition, several parametric variations are performed to observe the effects of the geometrical dimensions on the absorption performance. The free space measurement method has been utilised to measure the responses of the fabricated structure, which are in good agreement with the simulated results. The proposed ultra‐thin (∼0.013λ0 thin corresponding to lowest absorption frequency) absorber is anticipated to be useful in various potential applications like stealth technology, electromagnetic interference, electromagnetic compatibility and wireless communication.
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