Timeline for Are the vertical sections of the Ackermann function primitive recursive?
Current License: CC BY-SA 4.0
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| Dec 14, 2020 at 10:24 | comment | added | Joel David Hamkins | Thank you very much for this soft answer, which is how I shall prefer to think about it. What I find curious about the situation is that this fact shows the typical emphasis on the diagonal is somewhat misplaced. One proves the horizontal sections are cofinal in PR by induction on the PR definitions, and then a soft argument like this can show that the vertical sections are too fast to be PR, without ever needing to consider the diagonal function specifically. | |
| Dec 12, 2020 at 17:18 | comment | added | gowers | Just seen that Gro-Tsen had already made essentially this argument. | |
| Dec 12, 2020 at 12:18 | history | answered | gowers | CC BY-SA 4.0 |