,
David P. Woodruff
,
Samson Zhou
Creative Commons Attribution 4.0 International license
Estimating the second frequency moment (F₂) of an underlying frequency vector is a fundamental problem in the streaming model. While recent work by Braverman and Zamir [STOC 2025] resolved the space complexity for constant failure probability in the insertion-only model, the optimal dependence on the failure parameter δ remained open. We close this gap by proving a tight high-probability lower bound of Ω(1/ε² log(1/δ) log(ε√n) / log(1/δ)) for (1±ε)-approximate F₂ estimation. The key challenge is the failure of prior multi-scale direct sum arguments under noise sensitivity. We introduce a noise-robust communication primitive, Exam Mostly Set Disjointness, and prove an Ω(m/t log(1/δ)) one-way lower bound. Embedding this into a multi-scale reduction yields the correct log(1/δ) dependence. We also give two complementary algorithms under natural structure assumptions. For streams with frequency bound B, we design a subsampling method using continuous F₀ tracking that replaces a log n factor with log B. For k-sparse streams, we develop a two-stage sketch using approximate Morris counters, replacing log n with log k and achieving a further log log m dependence on stream length.
@InProceedings{swartworth_et_al:LIPIcs.APPROX/RANDOM.2026.76,
author = {Swartworth, William and Woodruff, David P. and Zhou, Samson},
title = {{High Probability Streaming Lower Bounds for F₂ Estimation}},
booktitle = {Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2026)},
pages = {76:1--76:23},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-449-9},
ISSN = {1868-8969},
year = {2026},
volume = {392},
editor = {Singh, Mohit and Gur, Tom},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.APPROX/RANDOM.2026.76},
URN = {urn:nbn:de:0030-drops-277936},
doi = {10.4230/LIPIcs.APPROX/RANDOM.2026.76},
annote = {Keywords: streaming algorithms, lower bounds, moment estimation}
}