Martin's conjecture on degree-invariant functions

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Assume ZF+DC+AD\mathsf{ZF}+\mathsf{DC}+\mathsf{AD}. A function f:2ω→2ωf:2^\omega\to2^\omega is degree-invariant if A≡TBA\equiv_T B implies f(A)≡Tf(B)f(A)\equiv_T f(B), and a property holds almost everywhere (a.e.) with respect to Martin Measure when it holds on a cone. Write f′f' for the function defined by f′(x)=f(x)′f'(x)=f(x)', and let ≤m\leq_m denote the ordering used for the ranks of degree-invariant functions.

Martin's conjecture. Under these assumptions: (I) if ff is degree-invariant and is not increasing a.e., then ff is constant a.e.; and (II) ≤m\leq_m pre-well-orders the set of degree-invariant functions that are increasing a.e., with f′f' having ≤m\leq_m-rank α+1\alpha+1 whenever ff has ≤m\leq_m-rank α\alpha.

Martin's conjecture classifies degree-invariant functions by their almost-everywhere behavior, analogous to the classification of natural objects by iterates of the Turing jump. The source states that it remains an open problem, and notes that under suitable large-cardinal hypotheses it can be viewed as a conjecture about functions in L(R)L(\mathbb{R}).

References

Primary source

James Walsh, “On the hierarchy of natural theories”, arXiv:2106.05794 (2025).

Additional references

4 papers in this index state this conjecture (2014–2021). The statement above is taken from the most recent of them; the others are arXiv:2004.00174, arXiv:1907.10766, arXiv:1410.1052.

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