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Characterizing Sidon sets by interpolation properties of subsets

Colin C. Graham, Kathryn E. Hare (2008)

Colloquium Mathematicae

Pisier's characterization of Sidon sets as containing proportional-sized quasi-independent subsets is given a sharper form for groups with only a finite number of elements having orders a power of 2. No such improvement is possible for a general Sidon subset of a group having an infinite number of elements of order 2. The method used also gives several sharper forms of Ramsey's characterization of Sidon sets as containing proportional-sized I₀-subsets in a uniform way, again in groups containing...

Complemented ideals of group algebras

Andrew Kepert (1994)

Studia Mathematica

The existence of a projection onto an ideal I of a commutative group algebra L 1 ( G ) depends on its hull Z(I) ⊆ Ĝ. Existing methods for constructing a projection onto I rely on a decomposition of Z(I) into simpler hulls, which are then reassembled one at a time, resulting in a chain of projections which can be composed to give a projection onto I. These methods are refined and examples are constructed to show that this approach does not work in general. Some answers are also given to previously asked...

Complex Unconditional Metric Approximation Property for C Λ ( ) spaces

Daniel Li (1996)

Studia Mathematica

We study the Complex Unconditional Metric Approximation Property for translation invariant spaces C Λ ( ) of continuous functions on the circle group. We show that although some “tiny” (Sidon) sets do not have this property, there are “big” sets Λ for which C Λ ( ) has (ℂ-UMAP); though these sets are such that L Λ ( ) contains functions which are not continuous, we show that there is a linear invariant lifting from these L Λ ( ) spaces into the Baire class 1 functions.

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