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Submitted on 21 Jan 2015

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addendum to: orthonormal bases of regular wavelets in

spaces of homogeneous type

Pascal Auscher, Tuomas Hytönen

To cite this version:

Pascal Auscher, Tuomas Hytönen. addendum to: orthonormal bases of regular wavelets in spaces

of homogeneous type. Applied and Computational Harmonic Analysis, Elsevier, 2015, pp.1-2.

�hal-01107797�

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ADDENDUM TO : ORTHONORMAL BASES OF REGULAR WAVELETS IN SPACES OF HOMOGENEOUS TYPE

PASCAL AUSCHER AND TUOMAS HYTÖNEN*

Abstract. We bring a precision to our cited work concerning the notion of “Borel measures”, as the choice among different existing definitions impacts on the validity of the results.

We wish to bring a precision to our work [1]. The same remarks apply to the follow-up article [2]. Proposition 4.5 in [1] states that if µ is a non-trivial Borel measure on a quasi-metric space X, finite on bounded Borel sets, and 1 ≤ p < ∞, then Hölder-η-continuous functions of bounded support are dense in Lp(µ), where η is the Hölder exponent of the splines constructed in [1].

Depending on the meaning of “Borel measure”, as different definitions can be found in the literature, the result is correct or wrong.

If µ is a σ-additive measure on the Borel σ-algebra, then the result with the given proof is correct, as the measurable sets coincide with the Borel sets. In that case, all our results are valid as stated.

However, if µ is an outer measure or a σ-additive measure on X for which the Borel sets are µ-measurable, then for the first sentence of the proof to be valid one must add the condition that for every µ-measurable set A (in the sense of Caratheodory for the outer measure case), there is a Borel set B ⊇ A such that µ(A) = µ(B), and the rest of the proof goes through. In [3] for example, Borel outer measures are called regular if this condition holds for all A (not necessarily µ-measurable). With such a definition of a Borel measure, this regularity condition should be added to our statements. Without regularity, the correct conclusion of Proposition 4.5 is density in the space of Lpfunctions having a Borel measurable representative. Thus, our wavelet representations

are valid for functions in this subspace and 1 < p < ∞. This is enough for many purposes. References

[1] Auscher, P. and Hytönen, T. Orthonormal bases of regular wavelets in spaces of homogeneous type, Appl. Comput. Harmon. Anal., 34 (2013), 266–296.

[2] Hytönen, T. and Tapiola, O. Almost Lipschitz-continuous wavelets in metric spaces via a new randomiza-tion of dyadic cubes, J. Approx. Theory, 185 (2014), 12–30.

[3] Mattila, P. Geometry of sets and measures in euclidean spaces". Cambridge Studies in Advanced Mathe-matics, 44. Cambridge University Press, Cambridge, 1995.

Laboratoire de Mathématiques, UMR 8628, Univ. Paris-Sud and CNRS, F-91405 Orsay E-mail address: pascal.auscher@math.u-psud.fr

Department of Mathematics and Statistics, P.O. Box 68, FI-00014 University of Helsinki, Finland E-mail address: tuomas.hytonen@helsinki.fi

2010 Mathematics Subject Classification. 42C40, 41A15, 30Lxx, 42B25. Key words and phrases. Quasi-metric space, Borel measure, spline, wavelet. *Corresponding author. Telephone: +358 2941 51430.

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