The "direct product problem" is a fundamental question in complexity theory which seeks to understand how the difficulty of computing a function on each of k independent inputs scales with k. We prove the following direct product theorem (DPT) for query complexity: if every T-query algorithm has success probability at most 1 - ε in computing the Boolean function f on input distribution μ, then for α ≤ 1, every αεTk-query algorithm has success probability at most (2αε(1 - ε))kin computing the fc-fold direct product f⊗kcorrectly on k independent inputs from μ. In light of examples due to Shaltiel, this statement gives an essentially optimal tradeoff between the query bound and the error probability. Using this DPT, we show that for an absolute constant α >; 0, the worst-case success probability of any αR2(f)k-query randomized algorithm for f⊗kfalls exponentially with k. The best previous statement of this type, due to Klauck, Spalek, and de Wolf, required a query bound of O(bs(f)k). Our proof technique involves defining and analyzing a collec tion of martingales associated with an algorithm attempting to solve f⊗k. Our method is quite general and yields a new XOR lemma and threshold DPT for the query model, as well as DPTs for the query complexity of learning tasks, search problems, and tasks involving interaction with dynamic entities. We also give a version of our DPT in which decision tree size is the resource of interest.The "direct product problem" is a fundamental question in complexity theory which seeks to understand how the difficulty of computing a function on each of k independent inputs scales with k. We prove the following direct product theorem (DPT) for query complexity: if every T-query algorithm has success probability at most 1 - ε in computing the Boolean function f on input distribution μ, then for α ≤ 1, every αεTk-query algorithm has success probability at most (2αε(1 - ε))kin computing the fc-fold direct product f⊗kcorrectly on k independent inputs from μ. In light of examples due to Shaltiel, this statement gives an essentially optimal tradeoff between the query bound and the error probability. Using this DPT, we show that for an absolute constant α >; 0, the worst-case success probability of any αR2(f)k-query randomized algorithm for f⊗kfalls exponentially with k. The best previous statement of this type, due to Klauck, Spalek, and de Wolf, required a query bound of O(bs(f)k). Our proof technique involves defining and analyzing a collec tion of martingales associated with an algorithm attempting to solve f⊗k. Our method is quite general and yields a new XOR lemma and threshold DPT for the query model, as well as DPTs for the query complexity of learning tasks, search problems, and tasks involving interaction with dynamic entities. We also give a version of our DPT in which decision tree size is the resource of interest.

arXiv:1005.0644IEEE Conference on Computational Complexity (CCC)Jun 1, 20115 citations

Quadratically Tight Relations for Randomized Query Complexity

Rahul Jain, Hartmut Klauck, Srijita Kundu +4 more

Theory of Computing SystemsAug 10, 20191 citation

Distributional Quantum Query Complexity

Shalev Ben-David, M. H. Ebtehaj

Quantum query complexity enjoys a variety of pleasing joint computation properties: for example, a composition theorem asserting $Q(f\circ g)=Θ(Q(f)Q(g))$ for all Boolean functions $f$ and $g$ ; a direct sum theorem asserting that computing $k$ copies of a function (or search problem) costs $Ω(k)$ times as much as the cost of computing one copy; and a direct product theorem asserting that for Boolean functions, even succeeding at the direct sum problem with exponentially small probability still requires $Ω(k)$ times the cost of computing one copy to bounded error. However, all of these results are strictly for worst-case quantum query complexity. For example, if we have a fixed distribution $μ$ over inputs, the direct sum theorem says nothing about the quantum query complexity of computing $k$ copies of $f$ when the input comes from the product distribution $μ^k$ instead of being worst-case. (Note that while a standard Yao-type minimax theorem guarantees a hard distribution for the direct sum problem, there's no guarantee that this hard distribution is a product distribution.) A similar problem occurs for the direct product theorem and the composition theorem: none of these results respect distributions. In this work, we give distributional joint computation lower bounds for the composition, direct sum, and direct product problems. Along the way, we introduce some new tools for handling quantum query lower bounds, including (a) a new ``multiplicative'' variant of the $γ_2$ norm (which we use in place of the multiplicative adversary method for proving the direct product theorem), and (b) a new ``Shaltiel-free'' measure of quantum query complexity, which we show characterizes the composition behavior of distributional quantum query complexity and satisfies pleasing properties.Quantum query complexity enjoys a variety of pleasing joint computation properties: for example, a composition theorem asserting $Q(f\circ g)=Θ(Q(f)Q(g))$ for all Boolean functions $f$ and $g$; a direct sum theorem asserting that computing $k$ copies of a function (or search problem) costs $Ω(k)$ times as much as the cost of computing one copy; and a direct product theorem asserting that for Boolean functions, even succeeding at the direct sum problem with exponentially small probability still requires $Ω(k)$ times the cost of computing one copy to bounded error. However, all of these results are strictly for worst-case quantum query complexity. For example, if we have a fixed distribution $μ$ over inputs, the direct sum theorem says nothing about the quantum query complexity of computing $k$ copies of $f$ when the input comes from the product distribution $μ^k$ instead of being worst-case. (Note that while a standard Yao-type minimax theorem guarantees a hard distribution for the direct sum problem, there's no guarantee that this hard distribution is a product distribution.) A similar problem occurs for the direct product theorem and the composition theorem: none of these results respect distributions. In this work, we give distributional joint computation lower bounds for the composition, direct sum, and direct product problems. Along the way, we introduce some new tools for handling quantum query lower bounds, including (a) a new ``multiplicative'' variant of the $γ_2$ norm (which we use in place of the multiplicative adversary method for proving the direct product theorem), and (b) a new ``Shaltiel-free'' measure of quantum query complexity, which we show characterizes the composition behavior of distributional quantum query complexity and satisfies pleasing properties.

quant-pharXiv:2610.06835Oct 6, 20260 citations

Direct Product Theorems for Randomized Query Complexity

Shalev Ben-David, Eric Blais

We establish two new direct product theorems for the randomized query complexity of Boolean functions. The first shows that computing n copies of a function f, even with a small success probability of $\gamma^{n}$ , requires $\Theta(n)$ times the maximum distributional query complexity of f with success parameter $\gamma$ . This result holds for all success parameters $\gamma$ , even when $\gamma$ is very close to 1/2 or to 1. As a result, it unifies and generalizes Drucker’s direct product theorem (2012) for $\gamma$ bounded away from $\frac{1}{2}$ and 1 as well as the strong direct sum theorem of Blais and Brody (2019) for $\gamma \approx 1-1 / n$ . The second establishes a general list decoding direct product theorem that captures many different variants of “partial computation” tasks related to the function $f^{n}$ consisting of n copies of f. Notably, our list decoding direct product theorem yields a new threshold direct product theorem and other new variants such as the labelled-threshold direct product theorem. Both of these direct product theorems are obtained by taking a new approach. Instead of directly analyzing the query complexity of algorithms, we introduce a new measure of complexity of functions that we call discounted score. We show that this measure satisfies a number of useful structural properties, including tensorization, that make it particularly suitable for the study of direct product questions.We establish two new direct product theorems for the randomized query complexity of Boolean functions. The first shows that computing n copies of a function f, even with a small success probability of $\gamma^{n}$, requires $\Theta(n)$ times the maximum distributional query complexity of f with success parameter $\gamma$. This result holds for all success parameters $\gamma$, even when $\gamma$ is very close to 1/2 or to 1. As a result, it unifies and generalizes Drucker’s direct product theorem (2012) for $\gamma$ bounded away from $\frac{1}{2}$ and 1 as well as the strong direct sum theorem of Blais and Brody (2019) for $\gamma \approx 1-1 / n$. The second establishes a general list decoding direct product theorem that captures many different variants of “partial computation” tasks related to the function $f^{n}$ consisting of n copies of f. Notably, our list decoding direct product theorem yields a new threshold direct product theorem and other new variants such as the labelled-threshold direct product theorem. Both of these direct product theorems are obtained by taking a new approach. Instead of directly analyzing the query complexity of algorithms, we introduce a new measure of complexity of functions that we call discounted score. We show that this measure satisfies a number of useful structural properties, including tensorization, that make it particularly suitable for the study of direct product questions.

Annual Symposium on Foundations of Computer ScienceDec 14, 20250 citations

Quantum query complexity enjoys a variety of pleasing joint computation properties: for example, a composition theorem asserting

Q(f\circ g)=Θ(Q(f)Q(g))

for all Boolean functions

f

and

g

; a direct sum theorem asserting that computing

k

copies of a function (or search problem) costs

Ω(k)

times as much as the cost of computing one copy; and a direct product theorem asserting that for Boolean functions, even succeeding at the direct sum problem with exponentially small probability still requires

Ω(k)

times the cost of computing one copy to bounded error. However, all of these results are strictly for worst-case quantum query complexity. For example, if we have a fixed distribution

μ

over inputs, the direct sum theorem says nothing about the quantum query complexity of computing

k

copies of

f

when the input comes from the product distribution

μ^k

instead of being worst-case. (Note that while a standard Yao-type minimax theorem guarantees a hard distribution for the direct sum problem, there's no guarantee that this hard distribution is a product distribution.) A similar problem occurs for the direct product theorem and the composition theorem: none of these results respect distributions. In this work, we give distributional joint computation lower bounds for the composition, direct sum, and direct product problems. Along the way, we introduce some new tools for handling quantum query lower bounds, including (a) a new ``multiplicative'' variant of the

γ_2

norm (which we use in place of the multiplicative adversary method for proving the direct product theorem), and (b) a new ``Shaltiel-free'' measure of quantum query complexity, which we show characterizes the composition behavior of distributional quantum query complexity and satisfies pleasing properties.

\gamma^{n}

, requires

\Theta(n)

times the maximum distributional query complexity of f with success parameter

\gamma

. This result holds for all success parameters

\gamma

, even when

\gamma

is very close to 1/2 or to 1. As a result, it unifies and generalizes Drucker’s direct product theorem (2012) for

\gamma

bounded away from

\frac{1}{2}

and 1 as well as the strong direct sum theorem of Blais and Brody (2019) for

\gamma \approx 1-1 / n

. The second establishes a general list decoding direct product theorem that captures many different variants of “partial computation” tasks related to the function

f^{n}

consisting of n copies of f. Notably, our list decoding direct product theorem yields a new threshold direct product theorem and other new variants such as the labelled-threshold direct product theorem. Both of these direct product theorems are obtained by taking a new approach. Instead of directly analyzing the query complexity of algorithms, we introduce a new measure of complexity of functions that we call discounted score. We show that this measure satisfies a number of useful structural properties, including tensorization, that make it particularly suitable for the study of direct product questions.