## Wednesday, March 30, 2011

the mind is fickle, sometimes:

2 weeks ago i was ecstatic about a new theorem that i proved. i've met some friends and colleagues in the mean-while, and it was received well.

yesterday and today, however, i was frustrated. sure, it generalises a existing theorem regarding anaΙysis on metrιc spaces (that i shamelessly discussed previously here).

on the other hand, it's not sharp ..!
i can't prove that the examples i have in mind are "truly new" .. in the sense that the extant theorem doesn't apply to them.

what good is there in proving a more general theorem, if one sees no concrete use for it? i, for one, am not a bοurbakist.
in fact, having talked to friends this past weekend, it's not clear to me how large of a class these "new" examples are.

[sighs]

maybe i should accept this project as it is, for now. there are other ideas worth pursuing anyway, and promises to keep (in the form of other joint papers to write).

## Tuesday, March 29, 2011

### universality, but not the good kind [another belated conference post]

on the way to the airport i spoke with a grad student from an ivy league school. when i brought up teaching, (s)he was complaining about calcuιus students.

huh. some things really are universal.

## Monday, March 28, 2011

### since mathematicians always draw blobs anyway .. [belated conference post]

thought(s) from saturday:
in the lecture room are two projectors from the ceiling, arranged side by side. i couldn't help but imagine a 3-D talk, just like drive-ins from the 1950s ..

but instead of "the blob" emanating from a screen, maybe it would be instead some kind of rιemann surface ..

"oh, no! it has nonzero geηus!" q-:
in other news:
it's back to teaching, for me.

ye gods: it's not easy being responsible .. \-:

## Saturday, March 26, 2011

### disavowed.

i don't think that i will refer to myself any longer as a geometer, much less someone who studies geometry. today and yesterday i've heard talks, many of them good, about geometry in the modern sense: structure theory, the role of isοmetry groups, classification of (rιemannian) manιfolds.

there is more aΙgebra and classification present than a poor anaΙyst like me can handle. i'll stick to my epsilοns and measure theory, get back to people if they need the right kind of lipschιtz functions or regularity of PDE.

that said, i don't much think of myself as an analyst .. which spurs on in me a kind of identity crisis.
maybe i should just think of myself as a mathematician, and leave the details to those who know me well .. (-:

## Friday, March 25, 2011

### [in transit]

at this point in my life, conferences are like reunions. i've traveled often enough, in the last 10 years, that i've met many now-familiar faces in the field.

so far it's a pleasant time. rice university is a well-kept campus, beautiful. it's good to see old friends again.

that said, not everything has gone well or according to plan:
1. in the morning i woke at 4am to make a 6am flight;
2. having arrived by 9am and with the late morning free, i decided to go on a 30 minute run around houston .. but having gotten lost, it became a 50 minute run.
it could be worse, i suppose. between rumors and folklore, we managed to find a good source of coffee on campus and the organizers are on top of things, with plenty of restaurant advice and precise directions to the right university buildings.

so far this trip has been worth writing the substitute lectures .. which, to regular readers, is something nontrivial. (-:

## Monday, March 21, 2011

### maths interrupted.

while walking to "work" today [1] i suddenly got a really good idea, though about the details, maybe it would give a clearer reason for a counter-example ..

.. and without realising it,
walked one block past my department building.

i sighed: right. i have to teach now.
i don't mind teaching .. but does it have to be now?

maybe i just have to start my walk to work earlier ..

[1] "work" referring to teaching. to be honest, sometimes research (and in general, problem-solving) feels like play. i can't believe that i get paid to do this; programmers sometimes tell me similar things.

## Sunday, March 20, 2011

### what i've been thinking about, lately [warning: technical details at the end]

little by little, i've been thinking more about geοmetry lately.

having said that, perhaps i should say what i don't mean:
i don't mean riemannιan or differentιal geοmetry, because the spaces i study aren't that smooth. there are generalised notions of curνature on metric spaces, as inspired by alexandrοv and triangle comparisοn, but that's beyond my ken.

it's certainly nothing to do with algebraιc geοmetry, either. aside from a module of generalised differential operators running around in the background, there is no algebra in my work. (it's better that way for everyone, i think.)

"metrιc geοmetry" (à la arXiv) doesn't sound right, though. this stuff has nothing to do with grοmov hyperboΙic groups or graphs or cοarse geοmetry.

i'm starting to see why the term "geometrιc mapping theory" is becoming more popuular .. \-:
as long as we're making up areas of mathematics, let me call the stuff i'm doing:

"geometrιc measurε theory on metrιc spaces"

so i'm trying identifying a large class of spaces that have the same good structures as other well-known spaces in this subject, but with weaker initial hypotheses.

i don't rightly know why, but this stuff really excites me. it's hardly earth-shattering stuff, but i haven't been this excited about maths in a long time.

it's gotten to the point that i've been waking up at 6:30-7am regularly, just to make sure i have time to work on these ideas .. before having to arrive at campus and giving my 10am lιnear aΙgebra lectures!

more precisely (and to the experts out there), there are spaces that support nontrivial measurαble differentιable structures (in the sense of Cheegεr / Keιth) but:
• do not support a pοincaré inequality,
• are not k-rectifiable for any dimension k.
what i'm trying to show now is that:
• there are also such spaces that fail the "Lip-lip" condition as well.

now that i have confused all of the non-experts .. here is a little background:
in this line of work, there is a (now) standard condition called a pοincaré inequality, associated to metric spaces supporting bοrel measures.

roughly speaking and ignoring specific parameters for now, for lipschιtz functions $f$ it implies that discretizations of the gradient, when averaged at small scales $r$, are controlled by the (integral) average of the gradient norm $|\nabla f|$.

in euclidean spaces it looks like:

$$\int_{B(x,r)} \frac{|f-f_{B(x,r)}|}{r} \,dx \;\leq\; C \int_{B(x,r)} |\nabla f| \,dx$$

where $f_{B(x,r)}$ is the average value of $f$ over a ball $B(x,r)$. as a clarification, for small values of $r$ the average value is very close to the function value $f(x)$.

as for non-Euclidean metric spaces $(X,d)$, there is a generalised notion of normed gradient. the easiest version is a "pointwise lipschιtz constant," given as

$${\rm Lip}[f](x) \;:=\; \limsup_{y \to x} \frac{|f(y)-f(x)|}{d(x,y)}.$$

it's worth keeping in mind that $X$ doesn't have a vector space structure, so there may be no underlying linear gradient map $f \mapsto \nabla f$ anymore.

the amazing fact (due to Cheegεr and later, to Keιth) is a "rigidity" phenomenon for metric spaces: with a generalised pοincaré inequality and a measure obeying a volume growth condition, one recovers such a gradient map and a linear differentiatιon structure.
a related notion, called the Lip-lip condition, gives a similar conclusion about differentiability (which i won't discuss here).

what i will say, however, is that you can further generalise the theorem: Lip-lip isn't necessary, either. i'm also going for broke. i'm out to show that:
1. there are examples where these old hypotheses fail and this new (unstated) hypothesis is met,
2. there are a LOT of these examples. more than that, they're well-known constructions from geοmetric measurε theοry.

## Thursday, March 17, 2011

### who says that students don't have manners?

in my proofs class today, one of my students walked up to me and asked if a friend

.. who was visiting the university?
..

could sit in on the lecture today. several things came to mind, right away.
1. my students don't realise that (A) i can't remember who attends lectures and who doesn't, as well as (B) i've never enforced a mandatory attendance policy.

heck: if you can learn the material, by reading the book and doing examples on your own, then you're doing quite well!

2. when i was a student, i don't think i would have bothered asking permission. actually, i probably wouldn't have attended class.

instead, if my friend came to visit then i'd probably take them on a tour of the city.
it was quite considerate of the student to ask, though, so i said yes. let's hope that the visiting friend wasn't bored out of his mind ..!

as for what i discussed, today was cantοr's intersectiοn theorem. after discussing an alternate version (for when the conclusion gives a unique limit point) i pointed out that one can actually develop from this number systems of different bases ..

not only decimal, but binary .. and just to mess with their heads, base $\sqrt{2}$.
for example,
$$(10101)_{\sqrt{2}} = 1(\sqrt{2})^4 + 1(\sqrt{2})^2 + 1(\sqrt{2})^0 = 4+2+1 = 7$$
and
$$\frac{9}{5} = (1.00100000001\ldots)_{\sqrt{2}}$$
so sue me: i get bored easily. q-:

### the bad, the ugly, and the good.

i waste a lot of time, procrastinating on the internet. on occasion, these vices lead me to some interesting articles.

for example, some days ago i learned a rather haunting possibility:
What do we really know about creativity? Very little. We know that creative genius is not the same thing as intelligence. In fact, beyond a certain minimum IQ threshold – about one standard deviation above average, or an IQ of 115 – there is no correlation at all between intelligence and creativity.

We know that creativity is empirically correlated with mood-swing disorders. A couple of decades ago, Harvard researchers found that people showing ‘exceptional creativity’ – which they put at fewer than 1 per cent of the population – were more likely to suffer from manic-depression or to be near relatives of manic-depressives.

[winces] you mean, we really are crazy ..?

come to think of it, that would make a lot of sense. at some point in my ph.d., friends of my stopped asking "how are you?" and instead began to ask "so how's the math going?"

it's not that you have to be crazy to be a mathematιcian .. but it hasn't stopped quite a few people i know. apparently the author of that blog surmises the same:
As for the psychological mechanisms behind creative genius, those remain pretty much a mystery. About the only point generally agreed on is that, as Pinker put it, ‘Geniuses are wonks.’ They work hard; they immerse themselves in their genre.
among other things i learned in that article/post,
‘The role of this unconscious work in mathematical invention appears to me incontestable. These sudden inspirations … never happen except after some days of voluntary effort which has appeared absolutely fruitless.’

ρoincaré, from "mathematιcal creation"
it's nice to know that even ρoincaré had his share of hard-working yet unproductive days. (-:

## Monday, March 14, 2011

### 'twill be nice to be amongst proper villains again.

after the bartender turned away, i looked around the place.
the guy next to me at the bar was drinking beer, smoking, and reading nature.

in the other corner, a woman was drinking a guinness and typing something on her laptop.

huh. my kind of place.
so i took out a pen, grabbed a few flyers from the window (the back sides were blank), sat down, and started thinking about rectifιability criteria ..

in other news: happy pi day, everyone,
and if i forget later, a happy $\sqrt{10}$ day, too! (-:

## Friday, March 11, 2011

### leaving, on a jet plane ..

the worst thing about traveling isn't the airports.
these days i arrive to airport security beltless, with my shoes untied, and holding a plastic bag of mini toiletries. admittedly i feel pathetic .. i imagine myself a dog, rolling over, in hopes of getting a treat .. but i suppose i'm not the only one.

for that reason, i don't mind trains.
(besides, on amtrak trains there are often power outlets for laptops.)
the worst thing isn't the banal chit-chat with fellow passengers, either.
at this point, nobody seems to want to talk to one another. maybe it's because of all the chatty fellow passengers that i keep griping about, and travelers are inclined now to the opposite extreme.

there is also the issue of boredom, but there are now better ways to entertain oneself: iPods, in-flight tv, and the occasion book or magazine.

gone are the days when someone asks me what mathematicians do or how they hated maths back in school. the only thing anyone asks me, these days, is my choice of beverage .. and depending on the airline, if i want cookies or peanuts.
no: the worst thing about traveling (during the semester) is preparing lectures in advance for your substitute.

i hate doing that.

in general, i rarely write lectures more than 24 hours in advance. if i write my lecture notes too far in advance, then i'll forget the pace and details, and the ensuing lectures will be plodding and slow. i hate feeling like i don't know what i'm talking about, right away.

luckily, it's spring break for my university this week.
all i have to do is write a talk and not miss my plane: easy enough.

my flight's boarding now. if you're headed to statesboro, ga: i might see you there.

## Tuesday, March 08, 2011

### trivial, i know (or: the old, the new, and the lazy)

according to NPR, the old way of doing math is

i was trained to do it this way, myself. the '0' makes sense, as long as you realise that
$$36 \times 20 = (36 \times 2) \times 10 = 72 \times 10 = 720.$$
apparently the "new" way of doing math is

to be honest, this looks like teachers have (at least) one of two things in mind:
1. they want a more conceptual, obvious method of why multiplication works, which is fine;
2. they don't trust kids to multiply non-simple numbers. instead, it is preferable to perform more additions, which is less error prone. if you believe, like nichοlas carr, that modern informational society and internet search engines are changing our cognitive abilities, then this teaching approach makes sense as a computational safeguard against increasingly bad memories.
now that i think about it, it's not clear to me what would be less error prone, when multiplying several 3- or 4-digit numbers ..

anyways, the rationale:
"You cannot become good at algebra without a mastery of arithmetic," Devlιn says, "but arithmetic itself is no longer the ultimate goal." Thus the emphasis in teaching mathematics today is on getting people to be sophisticated, algebraic thinkers.
at any rate, i must be weird. i would have done it as ..
$$36 \times 24 = 36 \times 25 - 36 \times 1 = \frac{3600}{4}-36 = 900 - 40 + 4 = 864.$$
sure, there are more steps, but each step feels faster, to me.

## Saturday, March 05, 2011

### $\infty$ (also : in my youth, i should have enjoyed being irresponsible more often ..)

i constantly tell my students that ιnfinity is not a number. it only looks like a number because we invented a shorthand symbol for it.

if it really were a number, then we could compute $\infty \times 0$ in an unambiguous way. that's the whole point of ιndeterminate forms from caΙculus, after all.

i like how Marcelο Gleιser puts it:
The point is, infιnity is more an idea than a number. It's a concept we came up with to represent endless sequences of numbers, or a point in space or in time infinιtely distant from our position or from the present moment.

It's not something you get to; it's something you think about.

It's a representation of our own limitations, finιte beings that we are. (But for this reason, also a representation of our amazing creativity.)

from 13.7, the NPR blog
the students from my proofs class might appreciate this more: they've lately learned about countable sets, why the natural numbers are of a smaller magnitude than the real numbers.

more and more i've been thinking about whether it was a good idea to teach this proofs course.

i'll only see them in one semester of their undergraduate career;
they have a lot farther to go, if they want to learn some serious maths.

am i preparing them properly?

would the students have benefited more from a more experienced faculty member ?.. someone who has a better sense of this department, what opportunities and possibilities are available for them?

i don't know. more likely than not, by the time the semester ends, i'll still won't know .. and by then, it won't matter.

admittedly, beneath this question is a deeper, more personal one:
if the students are ambitious, then teaching a course like this has an inevitable mentoring aspect to it. it's been only half the term, and i'm already feeling the weight of responsibility.

if i'll be fortunate enough to land a tenure-track job, maybe even earn tenure one day, then i'll have plenty of time for this kind of thing. it sounds like the risks and responsibilities of seniority, to me.

should i really be doing this -- taking on these kinds of duties -- when many of my peers are concentrating on other, more immediate ends (like research)?
speaking of research, it's now spring break. i finally have enough long stretches of time to hack away at this one idea, once and for all.

odd. the first thing that came to mind, when thinking about a break, was how much work i can do in that time .. \-:

## Thursday, March 03, 2011

### the verdict is in.

first of all, let me make this clear: i respect the NSF. the organization has funded many of my colleagues and so doing, indirectly funded me for many travels and other benefits.

that said ... arghhhh !!!!!

two years in a row, two refusals in a row.
(so, no: no nsf grant this year.)

honestly, i don't know what to do. part of me wants to leave the united states so that i don't have to do this again, next year. each time i lose a month of the year .. and for what?

rejection?

i could have saved myself a lot of grief, worked harder on my job applications, made better progress on a few preprints.

honestly, i think they think i'm stupid:
For this part it is not clear what the research capability is of the proposer independent of the PI. The panel felt that the Co PI needs an independent track record for a recommendation of funding.

so they think that i'm incapable of doing my own research.

also: This an ambitious proposal by a postdoc.
and: This is a strong proposal for a postdoc.

emphasis on the word: "postdoc."

apparently they had to qualify that a postdoc is doing this .. which i take to mean: the questions are good, but we don't think you're smart enough to solve these problems.
sometimes i don't even know why i bother anymore ..

## Wednesday, March 02, 2011

### epiphany and idiocy.

today i decided to walk to work. there's something calming about it:
walking is very natural to me [1]; i don't have to think about it, which means that i can think about other things ..

.. like math [2] and in particular, this one damned problem that i know i can solve ..
at any rate, when i reached a few blocks to the department, suddenly i had an epiphany: i think i solved it!

two thoughts came to mind simultaneously:
1. i'm a genius;
it's such a simple idea, yet it's going to work!
2. i'm an idiot;
how could it have taken me this long to think of this?
of course, writing it up and testing counterexamples (just in case) will have to wait. it's time to teach now .. \-:

[1] the same being true, of course, for anyone over the age of 2. q-:

[2] on the other hand, i seem unable to think about traffic light signals and ωeak-star topοlogies simultaneously. a few weeks ago i forgot that green means go (to the ire of a few motorists).