Fourier Growth of Parity Decision Trees

Authors Uma Girish , Avishay Tal , Kewen Wu

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Author Details

Uma Girish
  • Princeton University, NJ, USA
Avishay Tal
  • University of California at Berkeley, CA, USA
Kewen Wu
  • University of California at Berkeley, CA, USA


We thank anonymous reviewers for helpful comments.

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Uma Girish, Avishay Tal, and Kewen Wu. Fourier Growth of Parity Decision Trees. In 36th Computational Complexity Conference (CCC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 200, pp. 39:1-39:36, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


We prove that for every parity decision tree of depth d on n variables, the sum of absolute values of Fourier coefficients at level 𝓁 is at most d^{𝓁/2} ⋅ O(𝓁 ⋅ log(n))^𝓁. Our result is nearly tight for small values of 𝓁 and extends a previous Fourier bound for standard decision trees by Sherstov, Storozhenko, and Wu (STOC, 2021). As an application of our Fourier bounds, using the results of Bansal and Sinha (STOC, 2021), we show that the k-fold Forrelation problem has (randomized) parity decision tree complexity Ω̃(n^{1-1/k}), while having quantum query complexity ⌈ k/2⌉. Our proof follows a random-walk approach, analyzing the contribution of a random path in the decision tree to the level-𝓁 Fourier expression. To carry the argument, we apply a careful cleanup procedure to the parity decision tree, ensuring that the value of the random walk is bounded with high probability. We observe that step sizes for the level-𝓁 walks can be computed by the intermediate values of level ≤ 𝓁-1 walks, which calls for an inductive argument. Our approach differs from previous proofs of Tal (FOCS, 2020) and Sherstov, Storozhenko, and Wu (STOC, 2021) that relied on decompositions of the tree. In particular, for the special case of standard decision trees we view our proof as slightly simpler and more intuitive. In addition, we prove a similar bound for noisy decision trees of cost at most d - a model that was recently introduced by Ben-David and Blais (FOCS, 2020).

Subject Classification

ACM Subject Classification
  • Theory of computation → Oracles and decision trees
  • Theory of computation → Communication complexity
  • Theory of computation → Quantum complexity theory
  • Fourier analysis of Boolean functions
  • noisy decision tree
  • parity decision tree
  • query complexity


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