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On the uniqueness of uniform norms and C*-norms

100%

Dabhi P., Dedania H.

Studia Mathematica

2009

tom 191

nr 3

263-270

We prove that a semisimple, commutative Banach algebra has either exactly one uniform norm or infinitely many uniform norms; this answers a question asked by S. J. Bhatt and H. V. Dedania [Studia Math. 160 (2004)]. A similar result is proved for C*-norms on *-semisimple, commutative Banach *-algebras. These properties are preserved if the identity is adjoined. We also show that a commutative Beurling *-algebra L¹(G,ω) has exactly one uniform norm if and only if it has exactly one C*-norm; this is not true in arbitrary *-semisimple, commutative Banach *-algebras.

Beurling algebra analogues of theorems of Wiener-Lévy-Żelazko and Żelazko

81%

Bhatt S., Dabhi P., Dedania H.

Studia Mathematica

2009

tom 195

nr 3

219-225

Let 0 < p ≤ 1, let ω: ℤ → [1,∞) be a weight on ℤ and let f be a nowhere vanishing continuous function on the unit circle Γ whose Fourier series satisfies $∑_{n∈ℤ} |f̂(n)|^{p}ω(n) < ∞$. Then there exists a weight ν on ℤ such that $∑_{n∈ℤ} |\widehat{(1/f)}(n)|^{p} ν(n) < ∞$. Further, ν is non-constant if and only if ω is non-constant; and ν = ω if ω is non-quasianalytic. This includes the classical Wiener theorem (p = 1, ω = 1), Domar theorem (p = 1, ω is non-quasianalytic), Żelazko theorem (ω = 1) and a recent result of Bhatt and Dedania (p = 1). An analogue of the Lévy theorem at the present level of generality is also developed. Given a locally compact group G with a continuous weight ω and 0 < p < 1, the locally bounded space $L^{p}(G,ω)$ is closed under convolution if and only if G is discrete if and only if G admits an atom. This generalizes and refines another result of Żelazko.

Ograniczanie wyników

2 Studia Mathematica

2 Dabhi P.

2 Dedania H.

1 Bhatt S.

2 2009

Wyniki wyszukiwania

On the uniqueness of uniform norms and C*-norms

Beurling algebra analogues of theorems of Wiener-Lévy-Żelazko and Żelazko