Skip to main content
added 1 characters in body
Source Link

Ryll-Nardzewski theorems states that if $T$ is a countable complete theory, then $T$ is $\aleph_0$-categorical if and only if for every $n<\omega$ there are only finitely many formulaformulas $\varphi(x_1,\ldots,x_n)$ up to equivalence relative to $T$.

$T$ is a countable theory if it can be built in a countable language.

My question is: Is there a complete uncountable theory which is $\aleph_0$-categorical, but that for some $n<\omega$ there are infinitely many formulas $\varphi(x_1,\ldots,x_n)$ up to equivalence relative to $T$?

Thanks

Ryll-Nardzewski theorems states that if $T$ is a countable complete theory, then $T$ is $\aleph_0$-categorical if and only if for every $n<\omega$ there are only finitely many formula $\varphi(x_1,\ldots,x_n)$ up to equivalence relative to $T$.

$T$ is a countable theory if it can be built in a countable language.

My question is: Is there a complete uncountable theory which is $\aleph_0$-categorical, but that for some $n<\omega$ there are infinitely many formulas $\varphi(x_1,\ldots,x_n)$ up to equivalence relative to $T$?

Thanks

Ryll-Nardzewski theorems states that if $T$ is a countable complete theory, then $T$ is $\aleph_0$-categorical if and only if for every $n<\omega$ there are only finitely many formulas $\varphi(x_1,\ldots,x_n)$ up to equivalence relative to $T$.

$T$ is a countable theory if it can be built in a countable language.

My question is: Is there a complete uncountable theory which is $\aleph_0$-categorical, but that for some $n<\omega$ there are infinitely many formulas $\varphi(x_1,\ldots,x_n)$ up to equivalence relative to $T$?

Thanks

Added tag
Link
Tom Leinster
  • 27.7k
  • 5
  • 109
  • 159
Source Link

Why Ryll-Nardzewski theorem fails for uncountable theories?

Ryll-Nardzewski theorems states that if $T$ is a countable complete theory, then $T$ is $\aleph_0$-categorical if and only if for every $n<\omega$ there are only finitely many formula $\varphi(x_1,\ldots,x_n)$ up to equivalence relative to $T$.

$T$ is a countable theory if it can be built in a countable language.

My question is: Is there a complete uncountable theory which is $\aleph_0$-categorical, but that for some $n<\omega$ there are infinitely many formulas $\varphi(x_1,\ldots,x_n)$ up to equivalence relative to $T$?

Thanks