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Transactions of the American Mathematical Society

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The structure of $ \sigma$-ideals of compact sets

Authors: A. S. Kechris, A. Louveau and W. H. Woodin
Journal: Trans. Amer. Math. Soc. 301 (1987), 263-288
MSC: Primary 03E15; Secondary 28A05, 42A63
MathSciNet review: 879573
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Abstract: Motivated by problems in certain areas of analysis, like measure theory and harmonic analysis, where $ \sigma$-ideals of compact sets are encountered very often as notions of small or exceptional sets, we undertake in this paper a descriptive set theoretic study of $ \sigma$-ideals of compact sets in compact metrizable spaces. In the first part we study the complexity of such ideals, showing that the structural condition of being a $ \sigma$-ideal imposes severe definability restrictions. A typical instance is the dichotomy theorem, which states that $ \sigma$-ideals which are analytic or coanalytic must be actually either complete coanalytic or else $ {G_\delta}$. In the second part we discuss (generators or as we call them here) bases for $ \sigma$-ideals and in particular the problem of existence of Borel bases for coanalytic non-Borel $ \sigma$-ideals. We derive here a criterion for the nonexistence of such bases which has several applications. Finally in the third part we develop the connections of the definability properties of $ \sigma$-ideals with other structural properties, like the countable chain condition, etc.

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Article copyright: © Copyright 1987 American Mathematical Society

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