Superpolynomial Lower Bounds for Decision Tree Learning and Testing
- URL: http://arxiv.org/abs/2210.06375v1
- Date: Wed, 12 Oct 2022 16:25:48 GMT
- Title: Superpolynomial Lower Bounds for Decision Tree Learning and Testing
- Authors: Caleb Koch and Carmen Strassle and Li-Yang Tan
- Abstract summary: We prove, under randomized ETH, superpolynomial lower bounds for two basic problems.
Our results imply new lower bounds for distribution-free PAC learning and testing of decision trees.
We show our lower bound for learning decision trees can be improved to $nOmega(log s)$.
- Score: 5.117810469621253
- License: http://creativecommons.org/licenses/by/4.0/
- Abstract: We establish new hardness results for decision tree optimization problems,
adding to a line of work that dates back to Hyafil and Rivest in 1976. We
prove, under randomized ETH, superpolynomial lower bounds for two basic
problems: given an explicit representation of a function $f$ and a generator
for a distribution $\mathcal{D}$, construct a small decision tree approximator
for $f$ under $\mathcal{D}$, and decide if there is a small decision tree
approximator for $f$ under $\mathcal{D}$.
Our results imply new lower bounds for distribution-free PAC learning and
testing of decision trees, settings in which the algorithm only has restricted
access to $f$ and $\mathcal{D}$. Specifically, we show: $n$-variable size-$s$
decision trees cannot be properly PAC learned in time $n^{\tilde{O}(\log\log
s)}$, and depth-$d$ decision trees cannot be tested in time $\exp(d^{\,O(1)})$.
For learning, the previous best lower bound only ruled out
$\text{poly}(n)$-time algorithms (Alekhnovich, Braverman, Feldman, Klivans, and
Pitassi, 2009). For testing, recent work gives similar though incomparable
bounds in the setting where $f$ is random and $\mathcal{D}$ is nonexplicit
(Blais, Ferreira Pinto Jr., and Harms, 2021). Assuming a plausible conjecture
on the hardness of Set-Cover, we show our lower bound for learning decision
trees can be improved to $n^{\Omega(\log s)}$, matching the best known upper
bound of $n^{O(\log s)}$ due to Ehrenfeucht and Haussler (1989).
We obtain our results within a unified framework that leverages recent
progress in two lines of work: the inapproximability of Set-Cover and XOR
lemmas for query complexity. Our framework is versatile and yields results for
related concept classes such as juntas and DNF formulas.
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