Degrees bounding principles and universal instances in reverse mathematics

A Turing degree d bounds a principle P of reverse mathematics if every computable instance of P has a d-computable solution. P admits a universal instance if there exists a computable instance such that every solution bounds P. We prove that the stable version of the ascending descending sequence principle (SADS) as well as the stable version of the thin set theorem for pairs (STS(2)) do not admit a bound of low_2 degree. Therefore no principle between Ramsey's theorem for pairs RT22 and SADS or STS(2) admit a universal instance. We construct a low_2 degree bounding the Erd\H{o}s-Moser theorem (EM), thereby showing that previous argument does not hold for EM. Finally, we prove that the only Delta^0_2 degree bounding a stable version of the rainbow Ramsey theorem for pairs (SRRT22) is 0'. Hence no principle between the stable Ramsey theorem for pairs SRT22 and SRRT22 admit a universal instance. In particular the stable version of the Erd\H{o}s-Moser theorem does not admit one. It remains unknown whether EM admits a universal instance.Comment: 23 page

Dominating the Erdos-Moser theorem in reverse mathematics

The Erdos-Moser theorem (EM) states that every infinite tournament has an infinite transitive subtournament. This principle plays an important role in the understanding of the computational strength of Ramsey's theorem for pairs (RT^2_2) by providing an alternate proof of RT^2_2 in terms of EM and the ascending descending sequence principle (ADS). In this paper, we study the computational weakness of EM and construct a standard model (omega-model) of simultaneously EM, weak K\"onig's lemma and the cohesiveness principle, which is not a model of the atomic model theorem. This separation answers a question of Hirschfeldt, Shore and Slaman, and shows that the weakness of the Erdos-Moser theorem goes beyond the separation of EM from ADS proven by Lerman, Solomon and Towsner.Comment: 36 page

Models of Arithmetic and Subuniform Bounds for the Arithmetic Sets

It has been known for more than thirty years that the degree of a nonstandard model of true arithmetic is a subuniform upper bound for the arithmetic sets (suub). Here a notion of generic enumeration is presented with the property that the degree of such an enumeration is an suub but not the degree of a non-standard model of true arithmetic. This anwers a question posed in the literature

Actas de las XXXIV Jornadas de Automática

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