On query complexity measures and their relations for symmetric functions

TL;DR

This paper investigates various query complexity measures for symmetric functions, establishing their relationships, equivalences, and bounds, including new constructions and characterizations for quantum and classical complexities.

Contribution

It provides explicit constructions linking adversary methods, certificate complexity, and spectral sensitivity for symmetric functions, and characterizes quantum query complexity of Gap Majority.

Findings

Positive adversary and private coin certificate game complexities are indistinguishable for symmetric functions.

Spectral sensitivity equals positive adversary and approximate degree for all symmetric functions.

Quantum query complexity of Gap Majority is characterized, with bounds relating to noisy randomized complexity.

Abstract

The main reason for query model's prominence in complexity theory and quantum computing is the presence of concrete lower bounding techniques: polynomial and adversary method. There have been considerable efforts to give lower bounds using these methods, and to compare/relate them with other measures based on the decision tree. We explore the value of these lower bounds on quantum query complexity and their relation with other decision tree based complexity measures for the class of symmetric functions, arguably one of the most natural and basic sets of Boolean functions. We show an explicit construction for the dual of the positive adversary method and also of the square root of private coin certificate game complexity for any total symmetric function. This shows that the two values can't be distinguished for any symmetric function. Additionally, we show that the recently introduced measure of spectral sensitivity gives the same value as both positive adversary and approximate degree for every total symmetric Boolean function. Further, we look at the quantum query complexity of Gap Majority, a partial symmetric function. It has gained importance recently in regard to understanding the composition of randomized query complexity. We characterize the quantum query complexity of Gap Majority and show a lower bound on noisy randomized query complexity (Ben-David and Blais, FOCS 2020) in terms of quantum query complexity. Finally, we study how large certificate complexity and block sensitivity can be as compared to sensitivity for symmetric functions (even up to constant factors). We show tight separations, i.e., give upper bounds on possible separations and construct functions achieving the same.