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\begin{multline} \mathrm{Def}(X) := \Bigl\{ \{y \mid y\in X \text{ and } \Phi(y,z_1,\ldots,z_n) \text{ is true in }(X,\in) \} \mid \\ \Phi \text{ is a first order formula and } z_1,\ldots,z_n\in X\Bigr\}. \end{multline}

(Constructible Universe, Wikipedia)

I saw somewhere at stackexchange that $\Phi$ can be defined, but was not able to find it. So, is there anyway to define what such $\Phi$ would be?

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    Have you tried to open a *book*? Wikipedia is a good place to get an overview about things, but not to study. This site is not a supplement for trying to figure things on your own after working through the first part of Jech's *Set Theory*, for example.2012-09-18

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