transference principle fourier multiplier

Natasha willow

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19-03-2025

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The transference principle is a powerful tool in harmonic analysis, providing a crucial link between the boundedness properties of Fourier multipliers on different function spaces. It essentially states that if a multiplier is bounded on one space, under certain conditions, it's also bounded on another, related space. This article will explore the transference principle for Fourier multipliers, delving into its statement, proof techniques, and applications. Understanding this principle is fundamental to analyzing the behavior of Fourier multipliers across various settings.

Understanding Fourier Multipliers

Before diving into the transference principle itself, let's solidify our understanding of Fourier multipliers. A Fourier multiplier is an operator defined by its action on the Fourier transform of a function. Specifically, given a function f with Fourier transform f̂, a multiplier m acts as follows:

(T_mf)^(ξ) = m(ξ)f̂(ξ)

The function m is called the multiplier. The boundedness of T_m on various Lp spaces (or other function spaces) is a central question in harmonic analysis. Determining this boundedness often hinges on the properties of the multiplier m.

Statement of the Transference Principle

The transference principle, in its most basic form, connects the boundedness of a Fourier multiplier on the circle group (T) to its boundedness on the real line (R). Let's consider two groups: the circle group T (which can be identified with the interval [-π, π)) and the real line R. The principle states:

If m is a bounded Fourier multiplier on L^p(T), then under suitable conditions on m, it is also a bounded Fourier multiplier on L^p(R).

The "suitable conditions" often involve regularity properties of m, such as smoothness or decay at infinity. Different versions of the transference principle exist, varying slightly in these conditions and the specific L^p spaces involved.

Proof Techniques and Variations

The proof of the transference principle often relies on techniques from approximation theory and functional analysis. A common approach involves approximating the Fourier transform on R by periodizing the function and utilizing the boundedness on T. Then, a limiting argument is employed to establish the boundedness on R.

Several variations exist, depending on the specific assumptions made about the multiplier m. Some versions might weaken the regularity conditions on m, while others might extend the result to more general function spaces beyond L^p spaces.

Important Considerations

• Periodicity: The transference principle leverages the inherent periodicity of the circle group. This periodicity allows for a connection between the multiplier's action on functions defined on the circle and its action on functions on the real line.
• Approximation: The approximation techniques employed in the proof are crucial. Carefully constructed approximations are needed to ensure the convergence of the argument and the preservation of boundedness properties.
• Generalizations: The transference principle can be generalized to other locally compact abelian groups beyond T and R. The core idea remains the same: transferring boundedness properties between different groups.

Applications of the Transference Principle

The transference principle has numerous applications in harmonic analysis and related fields. Here are a few examples:

• Simplifying Boundedness Proofs: It allows us to prove the boundedness of a Fourier multiplier on R by first proving it on the simpler setting of T. This simplification often reduces the technical complexity of the proof.
• Extending Results: It allows us to extend results obtained for multipliers on one group to other groups, leading to broader applicability of theorems.
• Studying Convolution Operators: Fourier multipliers are closely related to convolution operators. The transference principle can aid in the analysis of the boundedness of convolution operators on different spaces.

Conclusion

The transference principle for Fourier multipliers is a fundamental tool in harmonic analysis, bridging the gap between the boundedness properties of multipliers on different groups. Understanding its statement, proof techniques, and applications is essential for researchers in this field. Its ability to simplify proofs and extend results makes it an invaluable asset in analyzing the behavior of Fourier multipliers across various settings. Further exploration into specific variations and generalizations of this principle can provide a deeper understanding of its power and reach within harmonic analysis.