On the randomised query complexity of composition

TL;DR

This paper establishes a tight lower bound on the randomized query complexity of composed relations and promise functions, advancing understanding of composition in the query model.

Contribution

It proves a general lower bound for the randomized query complexity of relation composition and demonstrates the bound's tightness with specific examples.

Findings

Lower bound: R(f∘g^n) ∈ Ω(R(f)·√R(g))

Existence of relations matching the bound, showing tightness

Improves previous bounds for total functions by a factor of √log n

Abstract

Let $f\subseteq\{0,1\}^n\times\Xi$ be a relation and $g:\{0,1\}^m\to\{0,1,*\}$ be a promise function. This work investigates the randomised query complexity of the relation $f\circ g^n\subseteq\{0,1\}^{m\cdot n}\times\Xi$, which can be viewed as one of the most general cases of composition in the query model (letting $g$ be a relation seems to result in a rather unnatural definition of $f\circ g^n$). We show that for every such $f$ and $g$, $$\mathcal R(f\circ g^n) \in \Omega(\mathcal R(f)\cdot\sqrt{\mathcal R(g)}),$$ where $\mathcal R$ denotes the randomised query complexity. On the other hand, we demonstrate a relation $f_0$ and a promise function $g_0$, such that $\mathcal R(f_0)\in\Theta(\sqrt n)$, $\mathcal R(g_0)\in\Theta(n)$ and $\mathcal R(f_0\circ g_0^n)\in\Theta(n)$ $-$ that is, our composition statement is tight. To the best of our knowledge, there was no known composition theorem for the randomised query complexity of relations or promise functions (and for the special case of total functions our lower bound gives multiplicative improvement of $\sqrt{\log n}$).