m-topped degrees

Abstract

This chapter assesses m-topped degrees. The notion of m-topped degrees comes from a general study of the interaction between Turing reducibility and stronger reducibilities among c.e. sets. For example, this study includes the contiguous degrees. A c.e. Turing degree d is m-topped if it contains a greatest degree among the many one degrees of c.e. sets in d. Such degrees were constructed in Downey and Jockusch. The dynamics of the cascading phenomenon occurring in the construction of m-topped degrees strongly resemble the dynamics of the embedding of the 1–3–1 lattice in the c.e. degrees. Similar dynamics occurred in the original construction of a noncomputable left–c.e. real with only computable presentations, which was discussed in the previous chapter.