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We investigate the complexity of several classical model theoretic theorems about prime and atomic models and omitting types. Some are provable in RCA0, others are equivalent to ACA0. One, that every atomic theory has an atomic model, is not provable in RCA0 but is incomparable with WKL0, more than 1 1 conservative over RCA0 and strictly weaker than all the combinatorial principles of Hirschfeldt and Shore (2007) that are not 1 1 conservative over RCA0. A prior- ity argument with Shore blocking shows that it is also 1 1 -conservative over B 2. We also provide a theorem provable by a finite injury priority argument that is conservative over I 1 but implies I 2 over B 2, and a type omitting theorem that is equivalent to the principle that for every X there is a set that is hyperimmune relative to X. Finally, we give a version of the atomic model theorem that is equiv- alent to the principle that for every X there is a set that is not recursive in X, and is thus in a sense the weakest possible natural principle not true in the !-model consisting of the recursive sets.

The atomic model theorem and type omitting