Suppose f:I -> R is differentiable on an open interval I of R. Let D be
the set of points where f is discontinuous. We know D is meager and,
therefore, has an empty interior. I have 2 questions, about which I
couldn't come to a conclusion:
a) Is it possible that D is dense in I? Since f has the intermediate
value property, it seems to me it can't, but I couldn't come to a
conclusion.
b) D can have positive Lebesgue measure, right?
Thank you.
Amanda
I think you meant "... where f' is discontinuous" in the
first paragraph and "Since f' has the ..." in the second
paragraph.
The continuity set of a derivative on an open interval J
is dense in J. In fact, the continuity set has cardinality c
in every subinterval of J. On the other hand, the discontinuity
set of a derivative can have the following properties --->
1. dense in the reals
2. cardinality c in every interval
3. positive measure (hence, not Riemann integrable)
4. positive measure in every interval
5. full measure in every interval (i.e. measure zero complement)
6. have a Hausdorff dimension zero complement
Briefly, a subset D of the reals is a discontinuity set for some
bounded derivative if and only if D is an F_sigma first category
set. Moreover, derivatives for which D is large are plentiful in
the sense of Baire category (use the sup norm on the collection
of bounded derivatives) -- For most bounded derivatives, the set D
is the complement of a measure zero set. Note that this is much
stronger than simply saying that D has positive measure (i.e. that
the derivative isn't Riemann integrable). Note also that D and
the complement of D, for any of these Baire-typical bounded
derivatives, gives a partition of the reals into a measure zero set
and a first category set. In 1993 Bernd Kirchheim strengthened
this by proving that, given any Hausdorff measure function h,
for most bounded derivatives the set D is the complement of
a set that has Hausdorff h-measure zero. (I seem to have
left out Kirchheim's result in the first post below.)
HISTORICAL ESSAY ON CONTINUITY OF DERIVATIVES
http://groups.google.com/group/sci.math/msg/814be41b1ea8c024
Proof that derivatives have the intermediate value property
http://mathforum.org/kb/thread.jspa?messageID=5259756
A couple of years ago I was hard at work on a revision of
the "historical essay" post above, but then I had to stop and
work on other things (such as my Fall classes, which began
before I had finished). I haven't gotten back to it since
then, but it's something I intend on returning to eventually.
The revision would be several times longer and more detailed,
and what I've done so far is over twice as long already.
Dave L. Renfro
> Hello
>
> Suppose f:I -> R is differentiable on an open interval I of R. Let D be
> the set of points where f is discontinuous.
You mean f' in that last line.
> We know D is meager and,
> therefore, has an empty interior. I have 2 questions, about which I
> couldn't come to a conclusion:
>
> a) Is it possible that D is dense in I?
Yes. A basic theorem here is: Suppose f_n is a sequence of
differentiable functions on [a,b]. If f_n, f_n' converge
uniformly to f, g resp. on [a,b], then f' = g on [a,b].
So given a countable set D = {d_1, ...} in R, we can choose
functions f_n, differentiable everywhere, such that f_n' is
continuous everywhere except at d_n, and such that |f_n|, |f_n'|
< 1/2^n on R. f = f_1 + f_2 + ... will be differentiable on R,
and f' will be discontinuous precisely on D. D of course could be
dense in R.
> Since f has the intermediate
> value property, it seems to me it can't, but I couldn't come to a
> conclusion.
>
> b) D can have positive Lebesgue measure, right?
Yes, it can have full measure. Meaning, we can have R = D U E,
where m(E) = 0. The idea would be to take D = U K_n, where the
K_n's are pairwise disjoint compact sets with no interior (aka
fat Cantor sets). If you set f_n(x) =
d(x,K_n)^2*sin(1/d(x,K_n))*2^(-n), and add them up, you'll get an
example. Almost. Maybe not quite. You'd have to play around with
it a little.
> Thank you.
> Amanda