,
Guangxu Yang
,
Jiapeng Zhang
Creative Commons Attribution 4.0 International license
We prove an Ω(n / k + k) communication lower bound on (k - 1)-round distributional complexity of the k-step pointer chasing problem under uniform input distribution, improving the Ω(n/k - klog n) lower bound due to Yehudayoff (Combinatorics Probability and Computing, 2020). Our lower bound almost matches the upper bound of Õ(n/k + k) communication by Nisan and Wigderson (STOC 91). As part of our approach, we put forth gadgetless lifting, a new framework that lifts lower bounds for a family of restricted protocols into lower bounds for general protocols. A key step in gadgetless lifting is choosing the appropriate definition of restricted protocols. In this paper, our definition of restricted protocols is inspired by the structure-vs-pseudorandomness decomposition by Göös, Pitassi, and Watson (FOCS 17) and Yang and Zhang (STOC 24). Previously, round-communication trade-offs were mainly obtained by round elimination and information complexity. Both methods have some barriers in some situations, and we believe gadgetless lifting could potentially address these barriers.
@InProceedings{mao_et_al:LIPIcs.ITCS.2025.75,
author = {Mao, Xinyu and Yang, Guangxu and Zhang, Jiapeng},
title = {{Gadgetless Lifting Beats Round Elimination: Improved Lower Bounds for Pointer Chasing}},
booktitle = {16th Innovations in Theoretical Computer Science Conference (ITCS 2025)},
pages = {75:1--75:14},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-361-4},
ISSN = {1868-8969},
year = {2025},
volume = {325},
editor = {Meka, Raghu},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2025.75},
URN = {urn:nbn:de:0030-drops-227038},
doi = {10.4230/LIPIcs.ITCS.2025.75},
annote = {Keywords: communication complexity, lifting theorems, pointer chasing}
}