Projective hierarchy

（重定向自Projective set）

In the mathematical field of descriptive set theory, a subset $A$ of a Polish space $X$ is projective if it is $\boldsymbol{\Sigma}^1_n$ for some positive integer $n$ . Here $A$ is

The choice of the Polish space $Y$ in the third clause above is not very important; it could be replaced in the definition by a fixed uncountable Polish space, say Baire space or Cantor space or the real line.

单词	Projective set
释义	Projective set 英语百科 Projective hierarchy （重定向自Projective set） In the mathematical field of descriptive set theory, a subset $A$ of a Polish space $X$ is projective if it is $\boldsymbol{\Sigma}^1_n$ for some positive integer $n$ . Here $A$ is The choice of the Polish space $Y$ in the third clause above is not very important; it could be replaced in the definition by a fixed uncountable Polish space, say Baire space or Cantor space or the real line.
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