TR06-061 Authors: Venkatesan Guruswami, Prasad Raghavendra

Publication: 5th May 2006 01:22

Downloads: 723

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Learning an unknown halfspace (also called a perceptron) from

labeled examples is one of the classic problems in machine learning.

In the noise-free case, when a halfspace consistent with all the

training examples exists, the problem can be solved in polynomial

time using linear programming. However, under the promise that a

halfspace consistent with a fraction (1-\eps) of the examples

exists (for some small constant \eps > 0), it was not known how to

efficiently find a halfspace that is correct on even 51% of the

examples. Nor was a hardness result that ruled out getting agreement

on more than 99.9% of the examples known.

In this work, we close this gap in our understanding, and prove that

even a tiny amount of worst-case noise makes the problem of

learning halfspaces intractable in a strong sense. Specifically,

for arbitrary \epsilon,\delta > 0, we prove that given a set of

examples-label pairs from the hypercube a fraction (1-\eps) of

which can be explained by a halfspace, it is NP-hard to find a

halfspace that correctly labels a fraction (1/2+\delta) of the

examples.

The hardness result is tight since it is trivial to get agreement on

1/2 the examples. In learning theory parlance, we prove that

weak proper agnostic learning of halfspaces is hard. This settles a

question that was raised by Blum et al in their work on

learning halfspaces in the presence of random classification

noise, and in some more recent works as well.

Along the way, we also obtain a strong hardness for another basic

computational problem: solving a linear system over the

rationals.