[lblume]

I have always wondered about what could be recovered if the antecedent (i.e. in this case the Riemann hypothesis) does actually turn out to be false. Are the theorems completely useless? Can we still infer some knowledge or use some techniques? Same applies to SETH and fine-grained complexity theory.

[daxfohl]

It depends. The most likely scenario would be that RH holds except in very specific conditions. Then, any dependent theorems would inherit the same conditions. In many cases, those conditions may not affect the dependent theorem, so they'd still be completely valid. In some cases, those conditions may make the dependent theorem useless, like if RH was "all numbers are even", and your theorem was "all numbers % 2 equal zero, because we know even numbers % 2 are zero and we assume RH", then the exception to RH "except odd numbers" would make your theorem devolve to "all numbers % 2 are zero except the odd ones, because we know even numbers % 2 are zero", which is obviously just a restatement of an existing statement.

In other cases, the new condition affects your theorem but doesn't completely invalidate it. So you can either accept that your theorem is weaker, or find other ways to strengthen it given the new condition.

That's all kind of abstract though. I'm not an expert on RH or what other important math depends on it holding up. That would be interesting to know.

[nicf]

I don't know enough about the RH examples to say what the answer is in that case. I'd be very interested in a perspective from someone who knows more than me!

In general, though, the answer to this question would depend on the specifics of the argument in question. Sometimes you might be able to salvage something; maybe there's some other setting where same methods work, or where some hypothesis analogous to the false one ends up holding, or something like that. But of course from a purely logical perspective, if I prove that P implies Q and P turns out to be false, I've learned nothing about Q.