A Complete Dichotomy for $L_\infty$ Permutation Removal

Source abstract

We study the removal problem for permutation patterns under the normalized $L_\infty$ earth mover's distance. We prove a complete dichotomy. Let $$\mathcal E =\{(12),(21),(132),(213),(231),(312)\}.$$ If $σ\in\mathcal E$, then every permutation of length $n$ that is $\varepsilon$-far from being $σ$-free contains at least $\varepsilon(n-1)$ copies of $σ$; for $0<\varepsilon\le 1/16$, this is sharp up to a constant factor for all sufficiently large $n$. If $σ\in S_k\setminus\mathcal E$, then for every sufficiently large $n$ there is a permutation of length $n$ that is $1/(8k)$-far from being $σ$-free and contains exactly one copy of $σ$. Thus the usual positive-density removal conclusion fails for every nontrivial permutation pattern, but the sparse behavior is completely characterized: the minimum number of copies is linear for exactly six patterns and is one for all others. Consequently, no canonical one-sided tester that rejects only after finding a forbidden copy can have query complexity independent of the permutation length.