Source: J. Symbolic Logic Volume 76, Issue 4
(2011), 1184-1210.
Let ℰΠ denote the collection of all Π01 classes,
ordered by inclusion. A collection of Turing degrees 𝒞 is
called invariant over ℰΠ if there is some collection
𝒮 of Π01 classes representing exactly the degrees
in 𝒞 such that 𝒮 is invariant under automorphisms
of ℰΠ. Herein we expand the known degree invariant classes
of ℰΠ, previously including only {0} and
the array noncomputable degrees, to include all highn and non-lown
degrees for n≥2. This is a corollary to a very general definability
result. The result is carried out in a substructure G of ℰΠ,
within which the techniques used model those used by Cholak and
Harrington [6] to obtain the same definability for
the c.e. sets. We work back and forth between G and ℰΠ
to show that this definability in G gives the desired degree invariance
over ℰΠ.
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