I was about to post Does base n have any Rotate-Left-Double numbers?. Kevin Cruijssen told me in the sandbox comment that he got a proof that significantly simplifies the problem. I posted it to main, excited to see the proof. (Seriously, I should've reconsidered posting to main at this point; it's my fault.) Then Kevin posted the proof along with a 6-byte 05AB1E answer
So essentially, the challenge text is asking for
Given the base \$n≥2\$, determine if there exists a Rotate-Left-Double number in base \$n\$.
but the actual task became
Given a positive integer \$n≥2\$, determine if it is not a member of A056469 (the number 2 and the numbers in the form of \$2^k+2\$).
which definitely sounds easier, and has a super-simple formula
n-2&n-3 (as pointed by xnor).
The question: How to resolve this situation?
It is not that uncommon to see that a challenge asking for a complex mathematical property boils down to a very simple task, sometimes so much that having such a challenge is pointless. If we identified that in the sandbox, we can redesign the challenge to something different. (Again, this particular incident is my fault posting it to main after the simplification is found.) However, it might slip through the minds of both the challenge writer and sandbox reviewers, and get found by someone else shortly after posting.
What can I do about it?
I can think of a few possibilities:
- Do nothing.
- Close as a dupe (if applicable). Otherwise, post a new challenge clearly asking for the actual task, and close the OP as a dupe of the new one.
- Close with a custom reason, and refine it again in the sandbox.
I don't want to get the challenge deleted, because there is at least one answer whose author put a lot of effort to prove such a property.