# Towards Optimal Moment Estimation in Streaming and Distributed Models

## TL;DR

This paper advances the understanding of space complexity for estimating moments in streaming models, removing the random order assumption for certain cases and providing new bounds and protocols for various related problems.

## Contribution

It presents new upper bounds for worst-case streams when estimating p-th moments for p in (0,1], and introduces communication-efficient protocols for p in (1,2], expanding the theoretical landscape.

## Key findings

- Achieves worst-case stream bounds matching randomized bounds for p in (0,1].
- Provides new communication protocols with low max-communication for p in (1,2].
- Shows that certain lower bounds cannot be proved via natural communication complexity approaches.

## Abstract

One of the oldest problems in the data stream model is to approximate the $p$-th moment $\|\mathcal{X}\|_p^p = \sum_{i=1}^n |\mathcal{X}_i|^p$ of an underlying vector $\mathcal{X} \in \mathbb{R}^n$, which is presented as a sequence of poly$(n)$ updates to its coordinates. Of particular interest is when $p \in (0,2]$. Although a tight space bound of $\Theta(\epsilon^{-2} \log n)$ bits is known for this problem when both positive and negative updates are allowed, surprisingly there is still a gap in the space complexity when all updates are positive. Specifically, the upper bound is $O(\epsilon^{-2} \log n)$ bits, while the lower bound is only $\Omega(\epsilon^{-2} + \log n)$ bits. Recently, an upper bound of $\tilde{O}(\epsilon^{-2} + \log n)$ bits was obtained assuming that the updates arrive in a random order. We show that for $p \in (0, 1]$, the random order assumption is not needed. Namely, we give an upper bound for worst-case streams of $\tilde{O}(\epsilon^{-2} + \log n)$ bits for estimating $\|\mathcal{X}\|_p^p$. Our techniques also give new upper bounds for estimating the empirical entropy in a stream. On the other hand, we show that for $p \in (1,2]$, in the natural coordinator and blackboard communication topologies, there is an $\tilde{O}(\epsilon^{-2})$ bit max-communication upper bound based on a randomized rounding scheme. Our protocols also give rise to protocols for heavy hitters and approximate matrix product. We generalize our results to arbitrary communication topologies $G$, obtaining an $\tilde{O}(\epsilon^{2} \log d)$ max-communication upper bound, where $d$ is the diameter of $G$. Interestingly, our upper bound rules out natural communication complexity-based approaches for proving an $\Omega(\epsilon^{-2} \log n)$ bit lower bound for $p \in (1,2]$ for streaming algorithms. In particular, any such lower bound must come from a topology with large diameter.