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单词 ExampleOfDefinableType
释义

example of definable type


Consider (𝐐,<) as a structureMathworldPlanetmath in a languagePlanetmathPlanetmath with one binary relationMathworldPlanetmath, which we interpret as the order.This is a universalPlanetmathPlanetmath, 0-categorical structure (see example of universal structure).

The theory of (𝐐,<) has quantifier eliminationMathworldPlanetmath, and so is o-minimal.Thus a type over the set 𝐐 is determined by the quantifier free formulas over 𝐐, which in turn are determined by the atomic formulas over 𝐐.An atomic formula in one variable over B is of the form x<b or x>b or x=b for some bB.Thus each 1-type over 𝐐 determines a Dedekind cut over 𝐐, and conversely a Dedekind cut determines a complete type over 𝐐.Let D(p):={a𝐐:x>ap}.

Thus there are two classes of type over 𝐐.

  1. 1.

    Ones where D(p) is of the form (-,a) or (-,a] for some a𝐐. It is clear that these are definable from the above discussion.

  2. 2.

    Ones where D(p) has no supremum in 𝐐. These are clearly not definable by o-minimality of 𝐐.

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更新时间:2025/5/3 18:28:13