For a complete theory with infinite models in a countable language, the following are equivalent: the theory is aleph-zero-categorical; every type space is finite; and for every there are only finitely many formulas in variables modulo the theory.
Every reduct of an aleph-zero-categorical theory to a sublanguage is aleph-zero-categorical. There can only be fewer formulas in each arity, so the finite-formula characterization in the Ryll-Nardzewski theorem is preserved.
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An omega-categorical theory is a concept from model theory, a branch of mathematical logic. A first-order theory is said to be \(\omega\)-categorical if it has exactly one countable model up to isomorphism. This means that if a theory is \(\omega\)-categorical, any two countable models of this theory will be structurally the same; they can be transformed into each other via a bijective mapping that preserves the relations and functions defined by the theory.