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The Art of Problem Solving
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| (no subject) |
[Nov. 8th, 2009|11:58 pm] |
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If this isn't allowed go ahead and delete it, I understand, but if there's anyone out there who knows statistics, has a sympathetic heart, and doesn't plan on sleeping, (it being midnight and all).. please let me know, catch me on msn= katiechillas@yahoo.com or comment on here, I'll be up for awhile. I have a test due in the morning, I have been going through tests, the book, etc. and i can't quite work out some problems on Type I and Type II errors as well as confidence intervals. Have mercy on a right-brained college kid if you will! thanks |
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| Seeking Reference Material |
[Nov. 8th, 2009|08:17 pm] |
Hi everyone,
I am seeking any reference material that eventually explains Hopf algebras. Really, I'd take anything that does a good job at explaining related topics starting with the definition of a module. I need something designed for a non-specialist.
Thanks in advance. |
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| Manifolds question-zilla! |
[Nov. 7th, 2009|09:39 pm] |
All right, so normally I wouldn't be posting like three questions at the same time, but I had swine flu this week (make sure you get the vaccine if you haven't already, because I promise you don't want the swine flu, it sucks) and couldn't go to class or meet with people. So with no further ado:
1) We're supposed to show that a simple Lie group is linear. In the hint sheet the professor gave us, he says, let C_g be the map that is conjugation by g, and let Ad(g) = T_1 C_g, the differential of this map. (I think this is called the adjoint representation or something.) I have no idea where this hint is going, and I don't see anything extra special about simple Lie groups that helps us buy this result, except perhaps that they are connected (because the identity component of a Lie group is a normal subgroup).
2) Next thing I'd like to show is that the differential of the inversion map (call it inv) is negative the identity. The hint sheet says to use the fact that exp is a homomorphism from lines through the origin in the Lie algebra to the Lie group. Certainly it's the case that exp(-v) = inv(exp(v)), and I think this is probably what we want, but I don't know how to get T_1 inv in there.
3) Last one for now I promise. Why would the differential of the determinant be trace?
Thanks for your help. I'm going to go hang my head in manifolds-induced shame now. |
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| math gre |
[Nov. 6th, 2009|02:51 pm] |
I'm re-taking the Math GRE tomorrow morning (ugh), and I want to take a couple peppermints. Does anyone know if small things such as this are permitted? I can't seem to find an answer to this specific question...
Edit: Well, I definitely did better this time, that much is for sure. I still don't really understand why graduate schools care if I can do 3 hours worth of hero-level integration, though.... |
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| Places to look for postdoc jobs |
[Nov. 3rd, 2009|12:17 am] |
Ok, so I'm pretty much finished with my PhD and have started to look for postdoc positions. So far I've looked at mathjobs.org and EIMS and will contact people in my field (harmonic analysis and operator theory) and ask them to inform me if any kind of postdoc jobs open up (this is for the departments that aren't advertising an open postdoc position, though I'll directly email people of interest who are in departments that are advertising, just to inform them that I'm applying.)
So the question is, where else should I look for postdoc jobs (or really just academic jobs in general) to apply to? |
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| Lp norms |
[Nov. 2nd, 2009|12:33 pm] |
I've seen the result all over the place, but no proof: The Lp-norm of a function f approaches the L-infty norm of f as p approaches infinity. Any illuminating clues?
EDIT: We assume f is in L^p and L^infty, so that it's in L^q for all q > p. |
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| more analysis.... |
[Nov. 1st, 2009|10:44 pm] |
Two question:
1. Let B(H,K) denote the space of bounded linear operators H --> K. Show B(H,K) is complete under the operator norm ||T|| = sup{ ||Th|| : h in H and ||h||≤1 }.
I seem to really struggle with this operator norm guy. Obviously I want to start with a Cauchy sequence of operators and show that it converges to some operator in the space.... But I'm just not clear on how to do all that
2. Let g be in L^1(R). The operator L: L^2(R) --> L^2(R) defined by Lf(x) = ∫ g(x-y)f(y)dy is a bounded linear operator.
My initial thought is to try to realize g(x-y) as the kernel of some integral operator, but I'm not sure how to go about doing that exactly.
Any thoughts?
Thanks |
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| Safety Schools? |
[Oct. 31st, 2009|12:54 pm] |
Hi folks. Long time lurker here, and today I have a question. What sort of graduate schools would fit as "safety schools" for me when applying to graduate school?
I'm currently an undergraduate in a school usually ranked in the top 25 so far as math departments go. I have an overall GPA over the last couple of years of 3.88 and a math GPA of 3.96. I haven't gotten my subject GRE scores back, but if I remember right I answered 55 questions on it and only remember feeling shaky about one of them, so my score will probably be somewhere around 800.
I've taken undergraduate courses in Advanced Calculus, Real Analysis, and Topology/Differential Geometry, I've taken a graduate course in Modern Algebra, and I'm currently taking graduate courses in Complex Analysis, Real Analysis, and Manifolds. I've made the Annual Deans List before and been given a "top junior in math" award. During the summer I participated in an REU program. Right now, I'm working as a TA. And I have my three recommendations lined up from good faculty.
The biggest deficiencies in my record are that I started out originally in community college, so my time at my current school will come to three years, and I have no time right now so I don't see a senior thesis on the horizon.
Maybe I'm being dense, but I don't feel comfortable judging which Ph.D. programs would be close to a sure thing when applying. University of North Carolina at Chapel Hill? Or Clemson University? What do you think?
Thanks in advance! |
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| image of a compact operator... |
[Oct. 30th, 2009|07:45 pm] |
Two posts in one day, sorry.
Is the image of a compact linear operator (say, from H to K) necessarily open or closed?
My instinct is that neither of these is necessarily true, but I'm having a tough time thinking of any counterexamples...
Any hints?
Thanks |
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| looking for an example |
[Oct. 30th, 2009|10:40 am] |
I was working through Fary's proof of the Fary-Milnor theorem, and in one of the theorems he proofs on the way, he includes the hypothesis that k(C_n) ≤ k(s).
To explain:
Given a curve closed C, the closed planar curve C_n is the projection of C onto a plane with normal vector n. The quantity k(s) is the curvature of C at a point s. So the statement above says that the total curvature of C_n is less than the curvature of C at each point.
In a footnote Fary says that this is necessary because it is possible to find a curve C and a projection C' of C such that k(C) < ∞ and k(C') = ∞; that is, the total curvature of C is finite, while the total curvature of C' is not. However, he gives no example nor any indication as to how we can find such a curve.
I've been trying to come up with an example of such a curve, but without any luck. If anyone here by chance knows an example, or just has any general thoughts on the matter, it'd be a tremendous help
Thanks! |
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| (no subject) |
[Oct. 29th, 2009|11:46 pm] |
Hi! I'd love help with a statistics problem, if anybody has time. Thanks in advance!
The problem reads:We have a certain population. The true but unknown proportion of units with some specific characteristic is pi = 0.70. We want a random sample of size "n" units to estimate pi. What is the minimum value of "n" for a sampling distribution of "p" reasonable approximated by a normal distribution?
As far as I understand, with a normal distribution, you get a standard deviation of 1. And the equation I have for standard deviation involving pi is: (pi*(1-pi))/n, where n = sample size.
But when I tried that:(pi*(1-pi))/n = 1
(pi*(1-pi)) = n
.7*(1-.7) = n = .21? Clearly you can't have a sample size of .21. So I must be doing something wrong.
If anybody could help me out, that'd be awesome! Thank you. |
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| Opinions, advice? |
[Oct. 29th, 2009|06:39 pm] |
Can someone "lose" their ability to think mathematically? Has there ever been any study done where anti-depressants or certain medications can interfere with one's ability to think clearly? The reason I ask is because I seem to have lost all math capability the past year.
Now, I don't know if I can attribute this loss to medication, depression, or lack of interest, but I used to LOVE mathematics. There was a time I could do mathematics ALL day. Now I feel less sure, and for some reason I don't WANT to study math anymore.
Can you believe this?
I know there is an anti-anxiety med (Buspar) that one can take to improve memory and cognitive functioning. The only drawback to it, though, is that you have to take it TWICE a day at exactly the same times.
What a crappy world we live in... |
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| more analysis |
[Oct. 18th, 2009|05:13 pm] |
1. Show that a multiplication operator satisfies Mφ2 = Mφ if and only if φ is a characteristic function.
The "only if" direction is straightforward, but I'm unclear how to go the other way
2. Define V: L^2([0,1]) --> L^2([0,1]) by (Vf)(x) = ∫0x f(y)dy. Compute the adjoint of this operator.
I've played around with this without much success. I feel like there's some simple manipulation that I'm just not seeing
Thanks! |
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| Math education programs |
[Oct. 16th, 2009|12:26 pm] |
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At the end of this school year, I'll be graduating with a master's degree in pure math. I want to go into teaching, though, particularly at an early college (algebra, trig, calculus, etc.) level. I am thus looking for good, reputable math education programs to apply to for a Ph.D. So far I've come up with UCSB's Center for Mathematical Inquiry, Michigan's IBL, Wisconsin, UCSD/SDSU's MSED, and Oregon State. (Texas would be really cool, but they want you to have like five years of teaching experience, which I don't.) Any other places you folks think I should look at? Thanks for your help! |
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| Central series |
[Oct. 13th, 2009|09:12 pm] |
We just covered Nilpotent groups, and upper/lower central series, and as an example we looked at D_{8}. The Dihedral group with 8 elements has the same upper central series as it has lower central series.
I was wondering if this was something special, when the UCS matches the LCS. D_{8} is a fairly simple example, and can't admit much, but in general, is there a nice consequence from the matching up? |
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| Curiosity |
[Oct. 13th, 2009|09:35 pm] |
1) Say you have a fibration F -> E -> B. This says that E is a B's worth of F's. For instance, R -> S1xR -> S1: a cylinder is a circle's worth of lines. It's clear here that you can reverse this to say that a cylinder is a line's worth of circles (S1 -> S1xR -> R), but is this always the case?
EDIT: Okay, so this is almost never the case. Any ideas on what conditions you'd need for this to be the case? It seems like trivial vector bundles would always work, but can we get any more general?
2) The Hölder inequality looks an awful lot like a generalization of the L2 version of Cauchy-Schwarz - which do you figure came first?
EDIT: After taking 23 seconds to wikipedia this, Cauchy published the sum version of the CSI (heh) in 1821, and Hölder happened in 1888/9. I wonder if CS influenced H? |
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| odd fractions |
[Oct. 13th, 2009|02:00 am] |
Somehow I have been laboring under the impression that the nonexistence of odd perfect numbers was proven a few years ago. Then I saw a talk two years ago, using analytic number theory (which I don't do), and I thought it gave a result that given distinct odd integers a_1, a_2, ... all greater than 1, the sum 1/a_1+1/a_2+... is never an integer. But it appears that I was incorrect, as there is no proof about odd perfect numbers yet and I doubt that people have missed this. Does anyone know a result like this, or a counterexample? What am I missing?
P.S. The correlation here is that the factors of an odd perfect number are odd, so another way to think about it is as the sum of 1/a (for each factor a of the number) is 1 or 2 depending on whether you allow a=1.
EDIT: Good, examples. If I'd realized there were any that small I'd have computer-searched myself. Thanks people! |
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| Question for Math students |
[Oct. 12th, 2009|08:04 pm] |
What motivates you to do your homework? A great professor? Love for the subject?
For me, it was a love for problem-solving (and some GREAT professors in the past). But now I'm taking Elementary Real Analysis, and the concepts are WAY different from applied mathematics. It looks interesting, but my professor is bland. He talks in a monotone voice, and sometimes I swear it seems like he's listening to voices in his head because he stares into space and starts laughing for no apparent reason. Also, his lectures are disjointed and incoherent. He's just...weird. I can't get motivated AT ALL.
In short, I am feeling extremely depressed. Am I just lazy? What do you do when you're stuck in a math rut? |
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| new to LaTeX |
[Oct. 12th, 2009|08:38 pm] |
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Is there a way to type the inverse exponent (-1)? Of course ^{-1} works, but it looks kind of big, specifically the negative sign. |
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| functional analysis |
[Oct. 11th, 2009|08:52 pm] |
If H is an infinite-dimensional Hilbert space, show that no orthonormal basis for H is a Hamel basis.
What I've done:
Suppose E = {e_1, e_2, ... } is an orthonormal basis and a Hamel basis for H. Take h in H. Then there are scalars a_1, a_2, ..., a_n in F and basis elements e_1, e_2, ..., e_n in E such that h = a_1e_1 + a_2e_2 + ... + a_ne_n.
But I'm unsure how to proceed. I obviously want to use the fact that every element h can be written as a finite linear combination of the e_i's; for each h there is some n such that it can be written as an n-term linear combination of basis elements, and so I'd like to do something like... take a maximum or sup of all such n's..... But this doesn't seem plausible. How do I know such a max or sup exists?
Anyway, any thoughts are welcome, thanks! |
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