## Three Analysis Questions: Accumulation

Here are three fun Analysis questions, really aimed at my students who are studying their first/second course on Real Analysis, but hopefully it’s of wider interest as well.

We begin at the beginning, with a couple of definitions.

Definition.A function has theIVT Propertyif, for all real numbers with and there exists such that .

So, using this terminology, the Intermediate Value Theorem is the statement that a continuous function has the IVT Property.

Definition.Let and let . We say that is aweak limitof at if

for all there exists such that . Naturally, we say thathas a weak limit somewhereif there exists such that is a weak-limit of at .We say that is an

accumulation pointof a real sequence if for all there is an infinite set such that for all we have .

**The Questions**

Question 1.

Show the existence or non-existence of a function which has the IVT Property but which is nowhere continuous.

Question 2.

Prove or disprove the claim that every function has a weak limit somewhere.

Question 3.

Find a real sequence such that every real number is an accumulation point of .

Where should we start? Perhaps my favourite of the three is the first one: to show the existence or non-existence of a function with the IVT property but which is nowhere continuous. There are several approaches we might take, beginning with the following lemma.

Lemma 1.Suppose that a function has the property that, for all with , the restriction of to the interval is surjective onto . Then has the IVT property.

Hopefully the proof of this lemma is quite clear: the IVT property asks, of such a function restricted to such an interval, that it attain certain values. So if attains *all* real values, then the property is evidently satisfied.

Okay…. So what?

Lemma 2.Suppose that a function has the property that, for all with , the restriction of to the interval is surjective onto . Then is continuous nowhere.

Proof.Let be given, and consider, for example, . For any , by assumption restricted to the open interval is surjective onto . In particular, the image under of is not a subset of .

Ah-ha! Now it’s clear why we’re interested in these sorts of function. Such a `*locally surjective function*‘ would be a nowhere continuous function which nevertheless has the IVT property. The big question now is: does any locally surjective function exist?

Well, YES. Such functions do exist, and there are several ways to find them. First we give a slightly abstract construction, which might seem a little unsatisfactory.

Method 1.Recall that the cardinality of is , and the cardinality of is also . We may choose a bijection.

Also, let be the natural map sending each real number to its coset modulo , namely . Note that is surjective. Thus, the composition is also a surjection. Indeed, by the density of the rationals in the reals, each proper open interval contains a representative of each coset of in . That is, for each , there exists such that . A consequence of this is that the restriction to of is still surjective onto . Finally, the restriction of to is still surjective onto , which shows that is locally surjective, as required.

Wonderful, that was easy. But it wasn’t particularly constructive. Perhaps it felt a little unsatisfactory to rely on the cardinalities of these sets. Is there a better way?

That’s where I’ll leave it for the moment. To be continued!

In the meantime, try to construct a locally surjective function by using the decimal expansion of each real number.

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