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Asaf Karagila
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Perhaps the solution lies in the Levy absoluteness theorem.

We define the Levy hierarchy of formulas as follows.

  1. $\Sigma_0,\Pi_0,\Delta_0$ are formulas in the language of set theory which only have bounded quantifiers.

  2. $\Sigma_{n+1}$ formulas equivalent to formulas of the form $\exists x\varphi$, where $\varphi$ is $\Pi_n$; and similarly $\Pi_{n+1}$ are formulas equivalent to $\forall x\varphi$ where $\varphi$ is $\Sigma_n$.

  3. $\Delta_n$ are formulas which are both $\Sigma_n$ and $\Pi_n$.

Now we have that if $M$ is a transitive class (set or otherwise), then $\Delta_0$ formulas are absolute between $M$ and $V$ (as long as $M$ includes all the parameters). Next we have that $\Sigma_1$ formulas are upwards absolute and $\Pi_1$ sentencesformulas are downwards formulasabsolute, so $\Delta_1$ formulas are absolute.

And so we have that $\Sigma_2$ formulas are upwards absolute and $\Pi_2$ formulas are downwards absolute.

Perhaps the solution lies in the Levy absoluteness theorem.

We define the Levy hierarchy of formulas as follows.

  1. $\Sigma_0,\Pi_0,\Delta_0$ are formulas in the language of set theory which only have bounded quantifiers.

  2. $\Sigma_{n+1}$ formulas equivalent to formulas of the form $\exists x\varphi$, where $\varphi$ is $\Pi_n$; and similarly $\Pi_{n+1}$ are formulas equivalent to $\forall x\varphi$ where $\varphi$ is $\Sigma_n$.

  3. $\Delta_n$ are formulas which are both $\Sigma_n$ and $\Pi_n$.

Now we have that if $M$ is a transitive class (set or otherwise), then $\Delta_0$ formulas are absolute between $M$ and $V$ (as long as $M$ includes all the parameters). Next we have that $\Sigma_1$ formulas are upwards absolute and $\Pi_1$ sentences are downwards formulas, so $\Delta_1$ formulas are absolute.

And so we have that $\Sigma_2$ formulas are upwards absolute and $\Pi_2$ formulas are downwards absolute.

Perhaps the solution lies in the Levy absoluteness theorem.

We define the Levy hierarchy of formulas as follows.

  1. $\Sigma_0,\Pi_0,\Delta_0$ are formulas in the language of set theory which only have bounded quantifiers.

  2. $\Sigma_{n+1}$ formulas equivalent to formulas of the form $\exists x\varphi$, where $\varphi$ is $\Pi_n$; and similarly $\Pi_{n+1}$ are formulas equivalent to $\forall x\varphi$ where $\varphi$ is $\Sigma_n$.

  3. $\Delta_n$ are formulas which are both $\Sigma_n$ and $\Pi_n$.

Now we have that if $M$ is a transitive class (set or otherwise), then $\Delta_0$ formulas are absolute between $M$ and $V$ (as long as $M$ includes all the parameters). Next we have that $\Sigma_1$ formulas are upwards absolute and $\Pi_1$ formulas are downwards absolute, so $\Delta_1$ formulas are absolute.

Source Link
Asaf Karagila
  • 39.7k
  • 8
  • 134
  • 282

Perhaps the solution lies in the Levy absoluteness theorem.

We define the Levy hierarchy of formulas as follows.

  1. $\Sigma_0,\Pi_0,\Delta_0$ are formulas in the language of set theory which only have bounded quantifiers.

  2. $\Sigma_{n+1}$ formulas equivalent to formulas of the form $\exists x\varphi$, where $\varphi$ is $\Pi_n$; and similarly $\Pi_{n+1}$ are formulas equivalent to $\forall x\varphi$ where $\varphi$ is $\Sigma_n$.

  3. $\Delta_n$ are formulas which are both $\Sigma_n$ and $\Pi_n$.

Now we have that if $M$ is a transitive class (set or otherwise), then $\Delta_0$ formulas are absolute between $M$ and $V$ (as long as $M$ includes all the parameters). Next we have that $\Sigma_1$ formulas are upwards absolute and $\Pi_1$ sentences are downwards formulas, so $\Delta_1$ formulas are absolute.

And so we have that $\Sigma_2$ formulas are upwards absolute and $\Pi_2$ formulas are downwards absolute.