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On the Kaczmarz algorithm of approximation in infinite-dimensional spaces

Stanisław Kwapień, Jan Mycielski (2001)

Studia Mathematica

The Kaczmarz algorithm of successive projections suggests the following concept. A sequence ( e k ) of unit vectors in a Hilbert space is said to be effective if for each vector x in the space the sequence (xₙ) converges to x where (xₙ) is defined inductively: x₀ = 0 and x = x n - 1 + α e , where α = x - x n - 1 , e . We prove the effectivity of some sequences in Hilbert spaces. We generalize the concept of effectivity to sequences of vectors in Banach spaces and we prove some results for this more general concept.

Outer factorization of operator valued weight functions on the torus

Ray Cheng (1994)

Studia Mathematica

An exact criterion is derived for an operator valued weight function W ( e i s , e i t ) on the torus to have a factorization W ( e i s , e i t ) = Φ ( e i s , e i t ) * Φ ( e i s , e i t ) , where the operator valued Fourier coefficients of Φ vanish outside of the Helson-Lowdenslager halfplane Λ = ( m , n ) 2 : m 1 ( 0 , n ) : n 0 , and Φ is “outer” in a related sense. The criterion is expressed in terms of a regularity condition on the weighted space L 2 ( W ) of vector valued functions on the torus. A logarithmic integrability test is also provided. The factor Φ is explicitly constructed in terms of Toeplitz operators...

Prediction of time series by statistical learning: general losses and fast rates

Pierre Alquier, Xiaoyin Li, Olivier Wintenberger (2013)

Dependence Modeling

We establish rates of convergences in statistical learning for time series forecasting. Using the PAC-Bayesian approach, slow rates of convergence √ d/n for the Gibbs estimator under the absolute loss were given in a previous work [7], where n is the sample size and d the dimension of the set of predictors. Under the same weak dependence conditions, we extend this result to any convex Lipschitz loss function. We also identify a condition on the parameter space that ensures similar rates for the...

Some remarks on duality of stationary sequences

L. Klotz, M. Riedel (2002)

Colloquium Mathematicae

The paper clarifies the connection between Urbanik's and Miamee and Pourahmadi's concepts of duality for univariate weakly stationary random sequences. Some of Urbanik's results are proved in an alternative way and at the same time generalized to the multivariate case.

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