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A Direct Sum Result for the Information Complexity of Learning
Proceedings of the 31st Conference On Learning Theory, PMLR 75:1547-1568, 2018.
Abstract
How many bits of information are required to PAC learn a class of hypotheses of VC dimension d? The mathematical setting we follow is that of Bassily et al., where the value of interest is the mutual information I(S;A(S)) between the input sample S and the hypothesis outputted by the learning algorithm A. We introduce a class of functions of VC dimension d over the domain X with information complexity at least Ω(dloglog|X|d) bits for any consistent and proper algorithm (deterministic or random). Bassily et al. proved a similar (but quantitatively weaker) result for the case d=1. The above result is in fact a special case of a more general phenomenon we explore. We define the notion of {\em information complexity} of a given class of functions \cH. Intuitively, it is the minimum amount of information that an algorithm for X must retain about its input to ensure consistency and properness. We prove a direct sum result for information complexity in this context; roughly speaking, the information complexity sums when combining several classes.