TR22-185 Authors: Arkadev Chattopadhyay, Yogesh Dahiya, Nikhil Mande, Jaikumar Radhakrishnan, Swagato Sanyal

Publication: 29th December 2022 20:58

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A classic result of Nisan [SICOMP '91] states that the deterministic decision tree depth complexity of every total Boolean function is at most the cube of its randomized decision tree depth complexity. The question whether randomness helps in significantly reducing the size of decision trees appears not to have been addressed. We show that the logarithm of the deterministic decision tree size complexity of every total Boolean function on $n$ input variables is at most the fourth power of the logarithm of its bounded-error randomized decision tree size complexity, ignoring a polylogarithmic factor in the input size. Our result has the following consequences:

1. The deterministic AND-OR query complexity of a total Boolean function is at most the fourth power of its randomize AND-OR query complexity, ignoring a $(\log n)^{O(1)}$ factor.

2. The deterministic AND (OR) query complexity of a total Boolean function is at most the cube of its randomized AND (OR) query complexity, ignoring a $(\log n)^{O(1)}$ factor. This answers a recent open question posed by Knop, Lovett, McGuire and Yuan [SIGACT News '21].

3. The notion of rank of a Boolean function was defined in a classic work of Ehrenfeucht and Haussler [Information and Computation'89] in the context of learning theory, and is characterized by the logarithm of decision tree size up to a logarithmic factor in the input size. Our results confirm a recent conjecture (ignoring a $(\log n)^{O(1)}$ factor) of Cornelissen, Mande and Patro [FSTTCS '22], that asserted the equivalence of randomized and deterministic analogs of rank, upto polynomial factors, for all total Boolean functions.

4. Combined with the above-mentioned work of Ehrenfeucht and Haussler, our result implies that the class of functions computable by randomized decision trees of polynomial size, is PAC-learnable in quasi-polynomial time.

To obtain our main result on decision tree size, we introduce the notion of block number of a Boolean function, which can be thought of as a counting analog of block sensitivity of a Boolean function that played a central role in Nisan's result mentioned above.