Martin's conjecture on degree-invariant functions
Assume . A function is degree-invariant if implies , and a property holds almost everywhere (a.e.) with respect to Martin Measure when it holds on a cone. Write for the function defined by , and let denote the ordering used for the ranks of degree-invariant functions.
Martin's conjecture. Under these assumptions: (I) if is degree-invariant and is not increasing a.e., then is constant a.e.; and (II) pre-well-orders the set of degree-invariant functions that are increasing a.e., with having -rank whenever has -rank .
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 .
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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