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This problem is a part of an exercise in Zygmund's Real Analysis. It reads:

Construct a measurable set $E$ of $[0,1]$ such that for every sub-interval I, both sets $E\cap I$ and $I\setminus E$ have positive measure.

The hint is to consider Cantor-type subset, but if consider a Cantor-type set, there must exist a sub-interval I belong to E. So, it mean no interval should be in E. Any way if I belong to E,then $|E\cap I|<|E|=0$. So I can't really know how to use the hint.

Now my question is how can I modified it to met the condition?

The whole hint on book is "Take a Cantor-type subset of [0, 1] with positive measure, and on each subinterval of the complement of this sure sestruct another such set, and so on. The measures can be arranged so that the union of all the sets has the desired property."

Apple
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  • This has been answered many many times on MSE. – geetha290krm Sep 24 '23 at 04:27
  • I can't connect Borel set and the hint here Cantor-type subset. Thank you for reply. But I still not solve my problem. – Apple Sep 24 '23 at 07:06
  • Cantor type sets or fat Cantor sets will not work. It is not clear as to what the authors meant by Cantor type sets. You should simply ignore that hint. – geetha290krm Sep 24 '23 at 07:20
  • The whole hint on book is "Take a Cantor-type subset of [0, 1] with positive measure, and on each subinterval of the complement of this sure sestruct another such set, and so on. The measures can be arranged so that the union of all the sets has the desired property." – Apple Sep 24 '23 at 07:33
  • Yes, that works. I have heard of that proof before, but it is not very easy to show that it works. For me, at least, writing out a formal proof is hard. – geetha290krm Sep 24 '23 at 07:53

1 Answers1

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This is a rather broad interpretation of "Cantor-type." The basic idea is the following (letting $Q_i$ be an enumeration of the countably many nontrivial open intervals with rational endpoints):

  • We have countably many "positive requirements" $P_i\equiv m(E\cap Q_i)>0$. The obvious way to meet such a requirement is to put an interval into $E$.

  • We have countably many "negative requirements" $N_i\equiv m(E\setminus Q_i)>0$. The obvious way to meet such a requirement is to throw an interval out of $E$.

The obvious issue is that the action we take to satisfy one requirement might "injure" another, e.g. putting $(0,1)$ into $E$ to make $E\cap (-3, 17)$ have positive measure obviously causes a problem for $E\setminus ({1\over 3}, {1\over 2})$. One solution is to allow injury but only to a very controlled extent; e.g. in the above example we might allow ourselves to put a very tiny subinterval of $({1\over 3}, {1\over 2})$ back into $E$. More abstractly, we'll want to keep track of some numerical parameter during our construction which tells us how much we're allowed to "alter" the set being built.

The details are, in my opinion at least, fun to work out for oneself, so I've spoilered the construction I have in mind:

We'll define a sequence of sets $E_i$ (= the stage-$i$ approximation to $E$), as well as a sequence of "stability parameters" $\epsilon_i$, as follows. We start with $E_0=\emptyset$ and $\epsilon_0={m(Q_0)\over 4}$. Next, having defined $E_i$ and $\epsilon_i$, we now face the pair of requirements $P_i$ and $N_i$. We pick a pair of disjoint open subintervals $I,J$ of $Q_i$ with $m(I)=m(J)=\epsilon_i$. Of course this requires $\epsilon_i$ to be "small" relative to $Q_i$; that will be true by induction, and it's with this in mind that we set $$\epsilon_{i+1}=\min\{{\epsilon_i\over 2^{i+2}}, {m(Q_{i+1})\over 2^{i+2}}\}.$$ More obviously, we also set $E_{i+1}=E_i\cup I\setminus J$. Finally, we take the "limsup" of the sequence of sets we've built so far: $$E=\{x:\forall m\exists n>m(x\in E_n)\}.$$ (The "liminf" would also work, the point is just that we need to account for oscillation of membership in the $E_i$s as $i\rightarrow\infty$.) Since the sequence $(\epsilon_i)_{i\in\mathbb{N}}$ shrinks sufficiently rapidly, and is always small relative to the interval being looked at at a given stage, it's easy to see that all requirements are satisfied.

Noah Schweber
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  • This has been answered many many times on MSE. – geetha290krm Sep 24 '23 at 04:27
  • In fact, I found a large number of duplicates using approach0. – geetha290krm Sep 24 '23 at 04:32
  • Sorry, I can't understand $P_i$,$N_i$ defination and how they work in your structure. – Apple Sep 24 '23 at 07:12
  • @geetha290krm: This was asked 2 or 3 days ago by what is almost certainly the same person using a new account (essentially the same question, with follow-up comments that have very similar content and writing style), but it seems to have been deleted and I had not saved the URL so I don't know how to find it. The earlier version did not mention "Zygmund", but in 2 or 3 comments I said it appears to be from Measure and Integral by Wheeden/Zygmund, and I gave several MSE links to similar questions (including "Construction of a Borel set ..."), (continued) – Dave L. Renfro Sep 24 '23 at 10:13
  • and I said not to worry about "Borel" -- the set will be a countable union of closed nowhere dense sets, and I cited two answers at "Construction of a Borel set ..." that construct such a set in the manner of the hint given there/here. In fact, the wording given in that earlier question and in a comment here is the same as used by Exercise 25 in Chapter 3. FYI, I actually worked this exercise back in Fall 1982, as I had a 2-semester course from this book in 1982-1983 under this person. – Dave L. Renfro Sep 24 '23 at 10:19