# One-Way Communication Complexity and Non-Adaptive Decision Trees

### Abstract

We study the relationship between various one-way communication complexity measures of a composed function with the analogous decision tree complexity of the outer function. We consider two gadgets: the AND function on 2 inputs, and the Inner Product on a constant number of inputs. More generally, we show the following when the gadget is Inner Product on $2b$ input bits for all $b \geq 2$, denoted $\mathsf{IP}$.

• If $f$ is a total Boolean function that depends on all of its $n$ input bits, then the bounded-error one-way quantum communication complexity of $f \circ \mathsf{IP}$ equals $\Omega(n(b-1))$.
• If $f$ is a partial Boolean function, then the deterministic one-way communication complexity of $f \circ \mathsf{IP}$ is at least the non-adaptive decision tree complexity of $f$.

To prove our quantum lower bound, we first show a lower bound on the VC-dimension of $f \circ \mathsf{IP}$. We then appeal to a result of Klauck [STOC ‘00], which immediately yields our quantum lower bound. Our deterministic lower bound relies on a combinatorial result independently proven by Ahlswede and Khachatrian [Adv. Appl. Math. ‘98], and Frankl and Tokushige [Comb. ‘99].

It is known due to a result of Montanaro and Osborne [arXiv ‘09] that the deterministic one-way communication complexity of $f \circ \mathsf{XOR}$ equals the non-adaptive parity decision tree complexity of $f$. In contrast, we show the following when the inner gadget is the AND function on 2 input bits.

• There exists a function for which even the quantum non-adaptive AND decision tree complexity of $f$ is exponentially large in the deterministic one-way communication complexity of $f \circ \mathsf{AND}$.
• However, for symmetric functions $f$, the non-adaptive AND decision tree complexity of $f$ is at most quadratic in the (even two-way) communication complexity of $f \circ \mathsf{AND}$.

In view of the first bullet, a lower bound on non-adaptive AND decision tree complexity of $f$ does not lift to a lower bound on one-way communication complexity of $f \circ \mathsf{AND}$.

The proof of the first bullet above uses the well-studied Odd-Max-Bit function.

For the second bullet, we first observe a connection between the one-way communication complexity of $f$ and the Möbius sparsity of $f$, and then give a lower bound on the Möbius sparsity of symmetric functions. An upper bound on the non-adaptive AND decision tree complexity of symmetric functions follows implicitly from prior work on combinatorial group testing; for the sake of completeness, we include a proof of this result.

It is well known that the rank of the communication matrix of a function $F$ is an upper bound on its deterministic one-way communication complexity. This bound is known to be tight for some $F$. However, in our final result we show that this is not the case when $F = f \circ \mathsf{AND}$. More precisely we show that for all $f$, the deterministic one-way communication complexity of $F = f \circ \mathsf{AND}$ is at most $(\mathsf{rank}(M_{F}))(1 - \Omega(1))$, where $M_{F}$ denotes the communication matrix of $F$.

Type
Publication
STACS 2022
##### Suhail Sherif
###### Postdoctoral Researcher

I work in theoretical computer science, with a focus on query and communication complexity.