Finite dimensional approximation and Newton-based algorithm for stochastic approximation in Hilbert space

  • Authors:
  • Ankur A. Kulkarni;Vivek S. Borkar

  • Affiliations:
  • Department of Industrial and Enterprise Systems Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA;School of Technology and Computer Science, Tata Institute of Fundamental Research, Mumbai, India

  • Venue:
  • Automatica (Journal of IFAC)
  • Year:
  • 2009

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Abstract

This paper presents a finite dimensional approach to stochastic approximation in infinite dimensional Hilbert space. The problem was motivated by applications in the field of stochastic programming wherein we minimize a convex function defined on a Hilbert space. We define a finite dimensional approximation to the Hilbert space minimizer. A justification is provided for this finite dimensional approximation. Estimates of the dimensionality needed are also provided. The algorithm presented is a two time-scale Newton-based stochastic approximation scheme that lives in this finite dimensional space. Since the finite dimensional problem can be prohibitively large dimensional, we operate our Newton scheme in a projected, randomly chosen smaller dimensional subspace.