The situation we are now in with EDP is roughly this. We have a statement that would imply EDP and that seems easier to prove because it is a reasonably simple existential statement (there exists a decomposition with certain properties) rather than a universal statement (every function has unbounded discrepancy on HAPs). However, the existential statement seems itself to be hard, despite one’s initial expectation that it ought to be easier. So what do we do?

One tool that we have at our disposal is duality, which is what we used to convert the problem to an existential one in the first place. Now obviously we don’t want to apply duality twice and end up with the original problem, but, perhaps surprisingly, there are ways that applying duality twice could be useful.

Here are two such ways. The first is that you prove that a certain kind of decomposition would be sufficient to prove EDP. Then you argue that if such a decomposition exists, then a more restricted kind of decomposition must also exist. Dualizing again, one ends up with a new discrepancy problem that is different from the original one (though it will imply it). The second way is this: if it is not easy to write down a decomposition that works, then one wants to narrow down the search space. And one way of doing that is to prove rigorously that certain kinds of decompositions do not exist. And an efficient way of doing that is to use duality: that is, one finds a function with low discrepancy on the class of sets that one was hoping to use for the decomposition. Since this class is restricted, solving the discrepancy problem is easier than solving EDP (but this time it doesn’t imply EDP).

We have already had an example of the first use of dualizing twice. In this post I want to give in detail an example of the second.

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