The Spectre of Math

June 25, 2013

New Genius out (1.0.17)

Filed under: Hacking,Linux,Mathematics — jlebl @ 9:18 pm

Release a new Genius version (1.0.17) today.  See the website.

Main new thing is that it is using Cairo to draw now thanks to update to new GtkExtra.  At some point I should package up and document all the fixes/changes I’ve made to GtkExtra and submit them upstream, it seems that GtkExtra is now alive again.

May 30, 2013

Windows 8.1 innovations

Filed under: Linux,Technology — jlebl @ 4:35 pm

I’ve just briefly glanced at what’s new in Windows 8.1, probably because I am procrastinating right now, not because I am actually interested, not having any plan of ever using Windows ever again in my life.  It was really amusing that new features are apparently that you can open more than two applications at once and also resize them on screen.  So maybe Windows 8.1 will finally catch up to Windows 1.0.

I feel I’m taking crazy pills here, because almost everyone is really into this trend.  First make the interface really bare (but graphically intensive) to make it look like a smartphone.  Then reintroduce very restrictive versions of these features and tout them as great achievements.  Chrome OS also at some point caught up with the 80s and added stacking windows.  I wonder when Windows will finally get into the 80s.

And while basic features like window management appear an insurmountable challenge in the new “iphone app” mentality, putting up a clock on your screen eats more CPU/GPU and memory than it would take to guide a spacecraft to Mars and back.  Not even that long ago did a calculator application not require 3D acceleration to reasonably run.

Good thing I’m using XFCE here.  Let’s see how I start a calculator: My non-accelerated desktop means that if I press Ctrl-Alt-R (which is my shortcut for terminal … R used to stand for rxvt long time ago) I immediately get a terminal.  Then I type “genius” press enter, and I’ve optimized the startup so I get a prompt immediately, there is essentially zero wait.  I type in a calculation, and out pops the answer.  Now if I attempt to do this on my phone, or in GNOME using the native calculator, there are several long waits either when some random nonsense is loading just for me to start the app and then there are these useless animations taking up time.  So for example if I have a student in my office and want to just quickly (note the word quickly) calculate something about their grade, I would definitely not do it on my phone.  Even try starting the native terminal in GNOME, it’s a few seconds wait on my machine from the time I get to start it (click a button, or whatnot).  So much for accelerated desktop. Just trying to phone someone on my phone is often a lot longer process than it used to be in the 90s when I got my first cellphone.  There is hardly a lack of power in the phone, it is just that it is so skillfully wasted.

This younger generation today must be really really patient (compared to us old fogies … you know 35 is old when it comes to technology) to put up with this.

May 28, 2013

Computation update

Filed under: Hacking,Mathematics — jlebl @ 4:45 pm

On the scale of length of computations I’ve done, this probably counts as the longest so far.  If you look down a bit in the blog you’ll find the details.  I can now report that there is no degree 21 polynomial p(x,y) with positive coefficients, with exactly 12 monomials (the least it can have), such that p(x,y) = 1 whenever x+y=1, and such that xy is one of the monomials.  Now the conjecture is that there is only one such beast (up to switching variables), dropping the condition about xy, and the computation is well on its way to prove that.  That one monomial is a bit special since it appears in these sharp polynomials for a bunch of smaller degrees.  Anyway, a few more months and we’ll have the answer.

March 29, 2013

Math is a series of trivial observations

Filed under: Mathematics,Personal — jlebl @ 4:38 pm

Mathematical proof is essentially a series of completely trivial observations wrapped in complicated-sounding notation (not complicated on purpose hopefully). The trick is not to understand the proof once it is written, but to notice those trivial observations to write a proof in the first place. I think this is what’s sometimes discouraging people from research mathematics. You work for two weeks on something that feels like a very hard problem, and then the solution seems trivial once found. In my case there are two operations and a limit involved. And the things you are trying to bound are not continuous with respect to that limit, so you flail around trying to do all sorts of complicated schemes. Then last night I think … hey why not do these two operations in reverse. I get rid of the limit and the problem becomes almost trivial after a bit of linear algebra. It feels good. But on the other hand it feels like: Why didn’t I think of this two weeks ago.

March 11, 2013

Moving to Oklahoma

Filed under: Personal — jlebl @ 8:06 pm

So, moving to OSU in Stillwater in the fall. This entails for the first time in my life quitting a job before it sort of naturally ends. The consulting gigs don’t really count I don’t think, and Eazel laid me off in the process of going under, so that counts as natural end of a job. At Red Hat it was just a summer gig, so there was no expectation of staying longer. Then at SDSU and UCSD I was a grad student, so any employment also ended when I graduated. In Urbana, they did make me send a “resignation letter”, but my appointment ended anyway as I stayed the whole 3 years I was hired for. At UCSD then I was hired for 1 year and stayed for one year. So now … I was hired for 3 and I am leaving after 2. Six years of postdoc in 3 different places are enough I think. One can always keep changing jobs every few years, but it’s nice to know I don’t have to now, I just can if I want to.

February 7, 2013

The computation

Filed under: Hacking,Linux,Mathematics,Technology — jlebl @ 4:47 pm

So the computation has finished (actually a few days ago) for degree 19. I’ve only yesterday gotten around to finishing a short paper (addendum) to post to arxiv, which I’ve done yesterday, see arXiv:1302.1441. The really funky thing is that there are so many sharp polynomials in degree 19. Up to symmetry there are 16 in odd degrees up to degree 17, yet there are 13 in degree 19. And two of the new ones are symmetric, which is actually surprising, that seems it should be hard to achieve if you think about how they are constructed. There’s probably a bunch of interesting number theory that appears here. It should be fun to figure out what’s going on there.

This was the first time a paper of mine got reclassified to a different archive on arxiv. I put it into algebraic geometry because well, the motivation comes from geometry, but it got stuck into comutative algebra. Which actually makes a lot more sense. Especially since none of the motivation from geometry appears in this writeup.

Degree 21 has been running for about a week. It will probably be running for the next year or so at which point I really expect it to just spit out only one polynomial which is the group invariant one we already know about. Which would be also kind of funky since then there would be two degrees with as few polynomials as possible and in between there would be a degree with the most polynomials we have found so far in any degree.

January 21, 2013

The correct finite field does wonders

Filed under: Hacking,Mathematics,Technology — jlebl @ 5:26 pm

So the computation I was running since thanksgiving was getting nowhere (finding degree 19 polynomials which are equal to 1 on x+y=1, which have fewest possible terms (11) and which have only positive coefficients, we know that there are finitely many, the trick is to find them). My code didn’t really have good status updates. Well it does tell you exactly where it is, but it takes a bit of computation to really see how it’s progressing, so I didn’t do that until a few days ago since I thought it was going way too slow.

And low and behold, it was going way too slow. I computed that it should take another 3-7 years or so (different parts of the search space take different times so it’s a bit hard to estimate). That was a bummer. That was about 10 times longer than I thought it should be. At first I was a bit resigned to this reality, but the next day I started to look into it, and one thing I figured out after running a bunch of tests was that one shortcut I was using was never triggered. The idea is that we need to find when a certain matrix with integer coefficients has 1 dimensional nullspace. Doing the integer row reduction is done with gmp, and is reasonably fast. But since we do this many times over and most of these matrices are simply full rank (no null space), we don’t really need to do the whole computation. So what we can do (and this is a useful trick to know), is to first do all computation in some small finite field, e.g. do everything mod p for some small prime p. If a matrix is full rank mod p, it is full rank. The computation can be done rather quickly this way and you don’t even have to involve modulo computation, since all the possible computations you can just precompute first and just build up a big table, so instead of two multiplications, an addition, and finding the remainder, you just look up in a table. Anyway, that gets us quite a bit of a speed up.

Now the thing is that I was using mod 19, since that worked for lower degrees. One thing I forgot when I started the run (remember this was a few years since I looked at this code and ran it last time), is that the modulus cannot be the same as the degree. The matrices we need to work with have most terms divisible by the degree. So moding out by 19 essentially always made the matrix all 0 (except for a few 1s scattered around). So these matrices were essentially always singular and the shortcut never triggered. So after doing a useless mod 19 calculation we had to do the actual integer arithmetic. That’s why it was slow. Damnit!

Well the calculations were not wrong, I just did a lot more computation than needed. After a small amount of testing it seemed that mod 23 was a good finite field to proceed in, so I restarted the code. Suddenly 3-7 years turned into first estimating 90 days and after running things for a day, that turned into an estimate of 30 days.

Then I noticed one more thing (and Danny pointed this out too), that his code used symmetry and just threw out half of the nonsymmetric polynomials, since the computations are the same. I remembered that my code didn’t do it. It didn’t make much sense if the longest run we did was 5 days on one core (for code that is only ever run once or twice, small speedups are somewhat pointless). I implemented this idea and it seems to achieve 33% reduction in time (there’s still the checking for symmetry, and there are of course symemtric polynomials, so that’s probably close to where we can get). So anyway, I guess within 20 days we should have the answer.

After it finsihes, I still have one more speedup up my sleeve. It could be that I can do the row reduction really fast mod 2 by using binary operations (each row would be an unsigned int). Not sure what speedup I can achieve though, at best 90%, since that’s how many cases mod 2 catches. While mod 23 or so catches essentially everything. So the idea is to do mod 2, then mod 23, and only then if the matrix is still singular do the integer arithmetic. If the speedup is another 50%, and my most optimistic estimates hold, that would put degree 21 within the realm of possibility, though at least half a year on 4 cores. That’s is, within something I’d be willing to run.

So, the mood went from “I’ll probably give up n d=19 soon” to “maybe d=21 is possible”. All this just by using a different prime :)

December 19, 2012

Interesting calculation

Filed under: Mathematics,Teaching — jlebl @ 5:02 pm

An interesting back of the envelope calculation: UC Irvine is using the differential equations book as standard, that’s a couple of hundred students every quarter, and there are at least a few other places with large and small lectures using the book. A reasonable estimate at the current adoption, there are at least 1000 students every year who use the book. I recently checked on amazon how much does Boyce-DiPrima cost: the new edition is $184 (Yaikes!). It costs $110 just to rent for a semester (a used version was $165). That’s 110-184 thousand dollars a year saved by students, just because of one open book adopted in a couple of large lecture classes. Presumably, the adoption rate can only go up, so this number will go up, just from one of my books (savings from the real analysis book are going to be much smaller due to less students taking that kind of class).

Now I have nothing against the publishers, but they have their incentives wrong. Boyce-DiPrima is a fine book, but … There is really no reason to print big bulky books on expensive paper for these classes. Locally printed coursepacks or cheaply printed paperbacks are much more efficient. And the students might actually keep their books and they might help them along in other classes. Currently most students return their books as soon as they can to recover most of the cost. So if you’re teaching say a PDE class as I did this semester, you can hardly tell them to brush up on their calculus from their calculus book. They don’t have it anymore!

The incentive for me is to simply make the best book because I want to (makes me feel good, that’s I guess all I can expect). Since I make almost no money on it, I don’t really have to inflate page count just to make it more expensive, or add color and pretty pictures just for the hell of it (it would make the book quite a bit more expensive). Plus the book is more accessible. I already know students use the web version even from their phones.

So anyway, I guess I’m providing at least 2 to almost 4 times my salary in free books.

Anyway, if you do want to buy the books (and I make $2.50 on each, yay! I made almost $400 so far, it’s mostly for making me feel good rather than making money), here are the links to lulu:

Real Analysis, $13.09 + shipping

Differential equations, $16.78 + shipping

Yes, I’m a fan of arbitrary looking prices. Actually the reason for those prices is that I simply set the price so that I get $2.50 from the book, so it’s the (cost of printing)+2.50+(lulu’s cut).

December 17, 2012

New versions of books and new genius

Filed under: Hacking,Mathematics,Teaching — jlebl @ 11:47 pm

So in the last two days I’ve put up new versions of both the differential equations textbook (3 new sections, and of course some fixes) and the real analysis textbook (fixes, plus 4 new exercises). And also I’ve made a new release of genius. Actually two of those I just did today when my students were taking their final. The nice things about proctoring tests with small upper division classes is that you can get stuff done. There is no cheating going on. There’s only a few questions, so I had over two hours to burn. Next semester will be quite different. I’ll have two calculus lectures with 250+ students each. Proctoring an exam for that many students is not at all a relaxing exercise (and then there’s the grading … ugh!)

December 9, 2012

Yet another new section in DE book

Filed under: Hacking,Mathematics,Teaching — jlebl @ 7:31 am

In trying to avoid bad mood and keep stress level down, people turn to hobbies. One of my hobbies is working on my textbooks, so I have written a new section on the Dirichlet problem for the Laplace equation in the circle for the differential equations book. See the draft section. The previous section 4.9 is OK, but the solution is far more natural in the circle in polar coordinates than in a square, that is we obtain

\displaystyle  u(r,\theta) = \frac{a_0}{2} + \sum_{n=1}^\infty a_n r^n \cos(n\theta) + b_n r^n \sin(n\theta)

And then we can derive the Poisson formula which is just cool. Also it’s a good example showing more complicated change of variables since we do it in polar, and also it shows a somewhat more complicated and different separation variables.

Part of the motivation was that I did this topic in my PDE class so I had lecture notes and it really felt right for that point in the book, even to leave it as reading to interested students. The other part is that I have been improving the graphing ability of genius so I can do polar coordinates for example:

dirich10speed

That’s the graph of the solution u(r,\theta) = r^{10} \cos(10\, \theta), showing that high frequency on the boundary means fast decay as you go closer to the center of your domain.

Though there is no UI for polar coordinates, there is just a function that allows you to plot arbitrary surface data now. Notice how it’s not graphed on a square grid, but above the disc. Also notice that internal rings have fewer points on them, that’s because I just compute fewer values at smaller radii, remember I am passing in arbitrary data, a list of tripples (x,y,z). This will be in version 1.0.16, which should come out end of next week sometime (have to let translators have a go at it). Actually the reason for doing this work on genius was not polar coordinates but showing numerical solutions in my PDE class. It’s just that one of my test cases was polar coordinates and so it just clicked and I thought: I have to write up this section, it’s just too cool to pass up and I can make the graphs now.

This brings the number of pages in the DE book to 315, and the number of exercises to 533. Yaikes! It’s become a beast. I’ll make the new version in a week or two so that it’s usuable for next semester (so if you have comments on the new section do let me know quickly).

I think now a two semester course could possibly be run out of the book. What’s going to be added in the new version is essentially about 5-6 lectures. At my speed the whole thing is now approximately 65 lectures, so a bit less than two semesters worth, but if you go just a tad slower (as many people do), do more examples, and if you factor in exams, reviews, quizzes, etc… it’s just right I think. You’ve got lots of room to spare if you want a two quarter course.

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