Titchmarsh convolution theorem

The Titchmarsh convolution theorem is named after Edward Charles Titchmarsh, a British mathematician. The theorem describes the properties of the support of the convolution of two functions.

Titchmarsh convolution theorem

E.C. Titchmarsh proved the following theorem in 1926:

If φ(t) and ψ(t) are integrable functions, such that

almost everywhere in the interval 0 < x < κ, then φ(t) = 0 almost everywhere in (0,λ), and ψ(t) = 0 almost everywhere in (0,μ), where .

This result, known as the Titchmarsh convolution theorem, could be restated in the following form:

Let . Then if the right-hand side is finite.

Similarly, if the right-hand side is finite.

This theorem essentially states that the well-known inclusion

is sharp at the boundary.

The higher-dimensional generalization in terms of the convex hull of the supports was proved by J.-L. Lions in 1951:

If , then .

Above, denotes the convex hull of the set. denotes the space of distributions with compact support.

The theorem lacks an elementary proof. The original proof by Titchmarsh is based on the Phragmén-Lindelöf principle, Jensen's inequality, Theorem of Carleman, and Theorem of Valiron. More proofs are contained in [Hörmander, Theorem 4.3.3] (Harmonic analysis style), [Yosida, Chapter VI] (Real analysis style), and [Levin, Lecture 16] (Complex analysis style).

References

• Titchmarsh, E.C. (1926). "The zeros of certain integral functions". Proceedings of the London Mathematical Society 25: 283–302. doi:10.1112/plms/s2-25.1.283. 

• Lions, J.-L. (1951). "Supports de produits de composition" (I and II). Les Comptes rendus de l'Académie des sciences 232: 1530–1532, 1622–1624. 

• Yosida, K. (1980). Functional Analysis. Grundlehren der Mathematischen Wissenschaften (Fundamental Principles of Mathematical Sciences), vol. 123 (6th ed.). Berlin: Springer-Verlag. 

• Hörmander, L. (1990). The Analysis of Linear Partial Differential Operators, I. Springer Study Edition (2nd ed.). Berlin: Springer-Verlag. 

• Levin, B. Ya. (1996). Lectures on Entire Functions. Translations of Mathematical Monographs, vol. 150. Providence, RI: American Mathematical Society.