Approximate Degree Composition for Recursive Functions

TL;DR

This paper proves that approximate degree composition holds for certain recursive Boolean functions, especially when the recursion depth is at least logarithmic in the logarithm of input size, using majority elimination techniques.

Contribution

The work establishes approximate degree composition for recursive functions with specific depth conditions, extending prior understanding to a broader class of functions.

Findings

Approximate degree composes for recursive functions with depth ; \, \\Omega(\,\log\log n)

Majority can be efficiently eliminated in recursive function compositions

Provides new techniques for analyzing approximate degree in recursive settings

Abstract

Determining the approximate degree composition for Boolean functions remains a significant unsolved problem in Boolean function complexity. In recent decades, researchers have concentrated on proving that approximate degree composes for special types of inner and outer functions. An important and extensively studied class of functions are the recursive functions, i.e.~functions obtained by composing a base function with itself a number of times. Let $h^d$ denote the standard $d$-fold composition of the base function $h$. The main result of this work is to show that the approximate degree composes if either of the following conditions holds: (I) The outer function $f:\{0,1\}^n\to \{0,1\}$ is a recursive function of the form $h^d$, with $h$ being any base function and $d= \Omega(\log\log n)$. (II) The inner function is a recursive function of the form $h^d$, with $h$ being any constant arity base function (other than AND and OR) and $d= \Omega(\log\log n)$, where $n$ is the arity of the outer function. In terms of proof techniques, we first observe that the lower bound for composition can be obtained by introducing majority in between the inner and the outer functions. We then show that majority can be \emph{efficiently eliminated} if the inner or outer function is a recursive function.