Abstract
We give an exposition of Herrmann’s proof that there is an infinite computable partial ordering with no infinite \(\Sigma ^0_2\) chains or antichains.
I am grateful to Eberhard Herrmann for finding the lovely proof on which this paper is based. I thank Rod Downey for his hospitality, collaboration, and support on my numerous trips to Wellington to work with him over the years. I also appreciate the opportunity to collaborate with him in many other places, including Urbana and Berkeley. Finally, I thank the Simons Foundation for supporting my travel to the meeting in New Zealand in 2017 in honor of his 60th birthday.
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Jockusch, C.G. (2017). Herrmann’s Beautiful Theorem on Computable Partial Orderings. In: Day, A., Fellows, M., Greenberg, N., Khoussainov, B., Melnikov, A., Rosamond, F. (eds) Computability and Complexity. Lecture Notes in Computer Science(), vol 10010. Springer, Cham. https://doi.org/10.1007/978-3-319-50062-1_10
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DOI: https://doi.org/10.1007/978-3-319-50062-1_10
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