> >A does not require demonstrating doc-rank problems map to n-day problems > Of course you have to, that's pretty much the whole operation of reducing A to B. You may be doing it implicitly but you're doing it for sure.

But the article isn’t mapping docrank to nday, nor is it claiming to reduce docrank to nday. Its choice of A and B is clear.

Nday didn’t have an understood solution to leverage like docrank did.

> one typically reduces the problem that one knows best (more general, known bounds solutions, etc...) into the one that you're studying

Wikipedia: “”” There are two main situations where we need to use reductions:

First, we find ourselves trying to solve a problem that is similar to a problem we've already solved. In these cases, often a quick way of solving the new problem is to transform each instance of the new problem into instances of the old problem, solve these using our existing solution, and then use these to obtain our final solution. This is perhaps the most obvious use of reductions. “””

This is what the article does, reducing the new nday problem into the known docrank one.

“”” Second: suppose we have a problem that we've proven is hard to solve, and we have a similar new problem. We might suspect that it is also hard to solve. We argue by contradiction: suppose the new problem is easy to solve. Then, if we can show that every instance of the old problem can be solved easily by transforming it into instances of the new problem and solving those, we have a contradiction. This establishes that the new problem is also hard. “””

This is your ‘transform A to B, this is called "reducing A to B", for some reason, and then you can say things like "B is at least as complex as A" and "I can solve some instances of B the way I solve the general case of A’. “Nday discovery is at least as complicated as document ranking” is obvious, though, which is why it’s not the subject of a blog post.