Finding monotone patterns in sublinear time

10/03/2019

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by   Omri Ben-Eliezer, et al.

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We study the problem of finding monotone subsequences in an array from the viewpoint of sublinear algorithms. For fixed k ∈N and ε > 0, we show that the non-adaptive query complexity of finding a length-k monotone subsequence of f [n] →R, assuming that f is ε-far from free of such subsequences, is Θ((log n)^log_2 k ). Prior to our work, the best algorithm for this problem, due to Newman, Rabinovich, Rajendraprasad, and Sohler (2017), made (log n)^O(k^2) non-adaptive queries; and the only lower bound known, of Ω(log n) queries for the case k = 2, followed from that on testing monotonicity due to Ergün, Kannan, Kumar, Rubinfeld, and Viswanathan (2000) and Fischer (2004).