@inproceedings{99755deab49544278c7f41cb4f990342,

title = "On the Hardness of Computing an Average Curve",

abstract = "We study the complexity of clustering curves under k-median and k-center objectives in the metric space of the Fr{\'e}chet distance and related distance measures. Building upon recent hardness results for the minimum-enclosing-ball problem under the Fr{\'e}chet distance, we show that also the 1-median problem is NP-hard. Furthermore, we show that the 1-median problem is W[1]-hard with the number of curves as parameter. We show this under the discrete and continuous Fr{\'e}chet and Dynamic Time Warping (DTW) distance. This yields an independent proof of an earlier result by Bulteau et al. from 2018 for a variant of DTW that uses squared distances, where the new proof is both simpler and more general. On the positive side, we give approximation algorithms for problem variants where the center curve may have complexity at most ℓ under the discrete Fr{\'e}chet distance. In particular, for fixed k, ℓ and ε, we give (1+ε)-approximation algorithms for the (k,ℓ)-median and (k,ℓ)-center objectives and a polynomial-time exact algorithm for the (k,ℓ)-center objective.",

keywords = "Curves, Clustering, Algorithms, Hardness, Approximation",

author = "Struijs, {Martijn A.C.} and Buchin, {Kevin A.} and Anne Driemel",

year = "2020",

month = jun,

day = "22",

doi = "10.4230/LIPIcs.SWAT.2020.19",

language = "English",

volume = "162",

series = "Leibniz International Proceedings in Informatics, LIPIcs",

publisher = "Schloss Dagstuhl - Leibniz-Zentrum f{\"u}r Informatik",

pages = "19:1--19:19",

editor = "Susanne Albers",

booktitle = "17th Scandinavian Symposium and Workshops on Algorithm Theory, SWAT 2020",

}