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Journal of Computer and System Sciences
Volume 67, Issue 2, September 2003, Pages 325-340
Special Issue on STOC 2002
 
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doi:10.1016/S0022-0000(03)00012-6    How to Cite or Link Using DOI (Opens New Window)
Copyright © 2003 Elsevier Inc. All rights reserved.

Fitting algebraic curves to noisy data

Sanjeev AroraE-mail The Corresponding Author, 1 and Subhash KhotCorresponding Author Contact Information, E-mail The Corresponding Author, 2

Department of Computer Science, Princeton University, 35 Olden Street, Princeton, NJ 08544, USA

Received 10 July 2002; 
revised 10 December 2002. 
Available online 19 July 2003.

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Abstract

We introduce the following problem which is motivated by applications in vision and pattern detection: We are given pairs of datapoints (x1,y1),(x2,y2),…,(xm,ym)set membership, variant[−1,1]×[−1,1], a noise parameter δ>0, a degree bound d, and a threshold ρ>0. We desire an algorithm that enlists every degree d polynomial h such that

(1)
|h(xi)−yi|less-than-or-equals, slantδ foratleast ρ fractionoftheindices i.
If δ=0, this is just the list decoding problem that has been popular in complexity theory and for which Sudan gave a poly(m,d) time algorithm. However, for δ>0, the problem as stated becomes ill-posed and one needs a careful reformulation (see the Introduction). We prove a few basic results about this (reformulated) problem. We show that the problem has no polynomial-time algorithm (our counterexample works for ρ=0.5). This is shown by exhibiting an instance of the problem where the number of solutions is as large as exp(d0.5−var epsilon) and every pair of solutions is far from each other in ℓ norm. On the algorithmic side, we give a rigorous analysis of a brute force algorithm that runs in exponential time. Also, in surprising contrast to our lowerbound, we give a polynomial-time algorithm for learning the polynomials assuming the data is generated using a mixture model in which the mixing weights are “nondegenerate.”

Author Keywords: Curve Fitting; Noisy polynomial reconstruction; List decoding; Learning theory; Vision

Article Outline

1. Introduction
1.1. The techniques used
2. Basic definitions
3. The basic problem
4. Learning mixtures of polynomials
4.1. Better algorithm for weak learning
5. Applications to machine vision and pattern detection
6. Mixture learning when mixing weights are nondegenerate
7. A counterexample
7.1. Counterexample for occluded curves
8. A stronger counterexample
9. Conclusions
Acknowledgements
References



Journal of Computer and System Sciences
Volume 67, Issue 2, September 2003, Pages 325-340
Special Issue on STOC 2002
 
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