I can prove that some known problem A is equivalent to another known problem B. It seems that this equivalence is new (I asked some senior people), but for the moment it does not help to progress in A or B. I wonder if this is still publisheable in a good journal.
Can a nice observation without practical consequences be published?
Sure, that kind of thing is publishable. There are plenty of papers that do exactly this.
How good of a journal it can get in is not answerable in the abstract. If you proved that RH is equivalent to Collatz, you can probably get it in the Annals. On the other hand, if the equivalence is sufficiently unsurprising or the problems are sufficiently boring the potential journal is unbounded below.
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