1,173 reputation
613
bio website cornellmath.wordpress.com
location Ithaca, NY
age 31
visits member for 4 years, 5 months
seen Oct 7 at 3:07
I was a graduate student at Cornell University studying harmonic analysis with Camil Muscalu. Now I'm a postdoc at Washington University in St. Louis.

May
31
comment A quick and elementary question from Hubbard's Teichmuller Theory : Volume I
Yep, got a surprising little more mileage out of that book. The OP and I should thank you for asking for the book back, otherwise the book wouldn't have been right next to me when I saw this question and this answer would never have happened.
May
31
answered A quick and elementary question from Hubbard's Teichmuller Theory : Volume I
May
31
comment Why is mechanical differentiation so hard to get right?
Well, it doesn't seem to be an issue with the derivative at all. Wolframalpha seems to believe that the given expression is $x^2/2$ when $x>0$, for instance.
May
30
comment Why is differentiating mechanics and integration art?
@Ryan: yeah, good point. As I was writing the previous comment, I realized it wasn't so tough to just get the right leading coefficient; I usually prove it just using the product rule ($x^n=x\times...\times x) and do the first few examples to avoid explicitly teaching induction because a lot of students really don't get the finite sum stuff. Although, I have done some stuff with the Riemann sums they seem to like if you phrase it correctly and avoid anything resembling induction :D
May
30
comment Why is differentiating mechanics and integration art?
@Deane A bit tongue in cheek, but: division of real numbers is harder than multiplication because it's secretly still multiplication, just with the additional step of inverting. Multiplication by $x$ is a continuous function, but multiplication by $x^{-1}$ is no longer continuous near zero. But if you look at, say, the circle group, multiplication and division are clearly equally difficult!
May
30
comment Why is differentiating mechanics and integration art?
Ryan --- he's saying that computing the derivative is simpler because it only depends on local information at a point. Integration depends on information about every point on a fixed interval, simultaneously. Thus differentiation is fundamentally simpler than integration, simply from the definitions. Doesn't it seem a lot easier to organize the cutting down of a forest one tree at a time than having to cut them all down simultaneously?
May
30
comment Why is differentiating mechanics and integration art?
That should be $n^{318}$. How shameful!
May
30
comment Why is differentiating mechanics and integration art?
@Ryan: ok, I see the distinction you're making. There is an argument one must make to be able to do constant step sizes (the function need be continuous). And I agree that in this case it comes down to computing those sums. I never remember the formulas for those sums aside from the first couple. Do you remember them for $n^318$, for example, or have any way to easily figure out what it is in such a case? :). I believe that business is rather involved (although all you really need is the coefficient of the highest power).
May
30
comment Why is differentiating mechanics and integration art?
1. u-sub is a statement about derivatives pushed into integration via the fundamental theorem of calculus: it's just a restatement of the chain rule. You can still prove a version of u-sub for absolutely continuous functions, but it takes more work. 2. You still need to check continuity at each point, but you can basically check the points one-by-one. But in any case, every absolutely continuous function is continuous, but plenty of continuous functions are not absolutely continuous. Indeed I would guess "most" continuous functions are not absolutely continuous. Isn't that an asymmetry?
May
30
answered Why is differentiating mechanics and integration art?
May
30
comment Why is differentiating mechanics and integration art?
Ryan --- I think all of my calculus students would disagree that Riemann sums are "easy" for polynomials. Indeed, I would have to agree with them! The computation only really becomes simple when you apply the fundamental theorem of calculus.
May
15
answered A Bijection Between the Reals and Infinite Binary Strings
May
12
answered Best online mathematics videos?
May
1
awarded  Yearling
Dec
1
comment Ingenuity in mathematics
That's interesting to hear, Elizabeth. I've almost always had the opposite experience, where they don't believe me and spend a great deal of energy arguing their point.
Nov
30
comment What is the difference between hard and soft analysis?
Mariano, the most likely reason people find your comment downvotable is that barbarous is essentially a synonym for barbaric in English, i.e. savage, violent, cruel, brutal, uncivilized, and uncultured. So, the obvious interpretation of your statement is something to the effect of "wouldn't it be nice if someone translated that ugly German text into a language actually spoken by civilized people?" I assume this was not your intention, of course! It is also somewhat common for people to say that German is an ugly language, probably a sentiment that has carried over from the 1940s.
Nov
27
awarded  Nice Answer
Nov
26
answered Nice Classes of Non-Closable Operators
Nov
20
comment What is the difference between hard and soft analysis?
I'm not exactly sure what you mean that this is a probabilistic argument. One can phrase it in a probabilistic way, but really it boils down to a hard linear algebra problem since the Markov process has finitely many states (the measures here are just vectors/functions). The statement that convergence happens is just the Perron-Frobenius Theorem. It is rather convenient to use probabilistic methods in the hard analysis of shuffling, though. I think this example rather clearly demonstrates the struggle between generality and quantitativeness. By the way, I myself happen to be an analyst...
Nov
20
answered What is the difference between hard and soft analysis?