Low-weight halfspaces for sparse boolean vectors

  • Authors:
  • Philip M. Long;Rocco A. Servedio

  • Affiliations:
  • NEC Labs, Cupertino, CA, USA;Columbia University, New York, NY, USA

  • Venue:
  • Proceedings of the 4th conference on Innovations in Theoretical Computer Science
  • Year:
  • 2013

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Abstract

For S ⊆ {0,1}n, a Boolean function f: S - {-1,1} is a halfspace over S if there exist w ∈ Rn and θ ∈ R such that f(x)=sign(w ⋅ x - θ) for all x ∈ S. We give bounds on the size of integer weights w1,...,wn ∈ Z that are required to represent halfspaces over Hamming balls centered at 0n, i.e. halfspaces over S ={0,1}n≤ k = {x ∈ {0,1}n : x1 + ⋅⋅⋅ + xn ≤ k}. Such weight bounds for halfspaces over Hamming balls have immediate consequences for the performance of learning algorithms in the increasingly common scenario of learning from very high-dimensional categorical examples which are such that only a small number of features are active in each example. We give upper and lower bounds on weight both for exact representation (when sign(w ⋅ x -θ) must equal f(x) for every x ∈ S) and for ε-approximate representation (when sign(w ⋅ x-θ) may disagree with f(x) for up to an ε fraction of points x ∈ S). Our results show that extremal bounds for exact representation are qualitatively rather similar whether the domain is all of {0,1}n or the Hamming ball {0,1}n≤ k, but extremal bounds for approximate representation are qualitatively very different between these two domains.