Improved Algorithm for Permutation Testing
Xiaojin Zhang
TL;DR
This work addresses efficient one-sided adaptive testing for the non-monotone forbidden pattern (1,3,2) in sequences. It introduces a monotone structure derived from the pattern’s third element and couples it with a fast binary-search-based strategy, aided by a simple $(1,2)$-pattern tester. The main result is an adaptive tester that distinguishes ε-far inputs from (1,3,2)-free ones with $O(ε^{-2}\log^4 n)$ queries, a substantial improvement over the previous $O(ε^{-7}\log^{26} n)$ bound. The approach sharpens the understanding of structure in forbidden-pattern testing and suggests pathways to generalize to broader pattern families with similar monotone-structure decompositions.
Abstract
For a permutation $π: [K]\rightarrow [K]$, a sequence $f: \{1,2,\cdots, n\}\rightarrow \mathbb R$ contains a $π$-pattern of size $K$, if there is a sequence of indices $(i_1, i_2, \cdots, i_K)$ ($i_1<i_2<\cdots<i_K$), satisfying that $f(i_a)<f(i_b)$ if $π(a)<π(b)$, for $a,b\in [K]$. Otherwise, $f$ is referred to as $π$-free. For the special case where $π= (1,2,\cdots, K)$, it is referred to as the monotone pattern. \cite{newman2017testing} initiated the study of testing $π$-freeness with one-sided error. They focused on two specific problems, testing the monotone permutations and the $(1,3,2)$ permutation. For the problem of testing monotone permutation $(1,2,\cdots,K)$, \cite{ben2019finding} improved the $(\log n)^{O(K^2)}$ non-adaptive query complexity of \cite{newman2017testing} to $O((\log n)^{\lfloor \log_{2} K\rfloor})$. Further, \cite{ben2019optimal} proposed an adaptive algorithm with $O(\log n)$ query complexity. However, no progress has yet been made on the problem of testing $(1,3,2)$-freeness. In this work, we present an adaptive algorithm for testing $(1,3,2)$-freeness. The query complexity of our algorithm is $O(ε^{-2}\log^4 n)$, which significantly improves over the $O(ε^{-7}\log^{26}n)$-query adaptive algorithm of \cite{newman2017testing}. This improvement is mainly achieved by the proposal of a new structure embedded in the patterns.