## Search found 69 matches

Sun Mar 04, 2012 12:38 pm UTC
Forum: Coding
Topic: I'm stupid. How to cast in linear programming setting?
Replies: 4
Views: 3849

### Re: I'm stupid. How to cast in linear programming setting?

Ah I see, thank you very much!
Wed Feb 29, 2012 1:57 am UTC
Forum: Coding
Topic: I'm stupid. How to cast in linear programming setting?
Replies: 4
Views: 3849

### Re: I'm stupid. How to cast in linear programming setting?

Thanks for moving, mods.
Tue Feb 28, 2012 9:42 pm UTC
Forum: Coding
Topic: I'm stupid. How to cast in linear programming setting?
Replies: 4
Views: 3849

### I'm stupid. How to cast in linear programming setting?

Either Matlab is trolling me or I'm having a massive brainfailure. Here's a homework question I'm trying to work out: We have 100 dollars that want to invest in gold. There are four suppliers with prizes 1/2, 1, 1/7 and 1/4 dollar per gram of gold. We want to maximize the amount of gold. Formulate t...
Wed Jan 26, 2011 12:43 pm UTC
Forum: Mathematics
Replies: 8
Views: 1561

### Re: Question about (un)conditional expectation

Oh well, I'm giving up on this question for now. Thanks anyway for your helpful input, D.B.!

 Oh, and everyone else who commented in this topic, ofcourse
Sat Jan 22, 2011 11:04 am UTC
Forum: Mathematics
Replies: 8
Views: 1561

### Re: Question about (un)conditional expectation

What is, for example, the difference between E[f] where is and is not known? Your notation is different enough to the kind I'm used to that I've been unsure about replying. However, I'll have a go. This could all be crap... I think you miss out a major part of the question when you don't tell u...
Thu Jan 20, 2011 10:32 am UTC
Forum: Mathematics
Replies: 8
Views: 1561

### Re: Question about (un)conditional expectation

Anyone?
Wed Jan 19, 2011 10:02 am UTC
Forum: Mathematics
Replies: 8
Views: 1561

### Re: Question about (un)conditional expectation

Thanks for your reply. I understand the conceptual difference between conditional and unconditional expectation. However, I'm having trouble seeing where and how this effects my computations. At the moment, all I got is (I'm gonna write f(x_0) = f and \hat{f}(x_0) = \hat{f} ): E_{\ma...
Mon Jan 17, 2011 4:10 pm UTC
Forum: Mathematics
Replies: 8
Views: 1561

I'm reading the book "The Elements of Statistical Learning" (2nd edition) by Hastie, Tibshirani and Friedman at the moment and I have a question about an exercise (exercise 2.7, if you care to read along). Given a sample of N pairs (x_i,y_i) drawn i.i.d. where x_i are distributed h(x&#...
Thu Dec 30, 2010 6:50 pm UTC
Forum: Mathematics
Topic: The Interesting Parts of Maths
Replies: 13
Views: 2915

### Re: The Interesting Parts of Maths

Is it just me or is there a really big gaping hole for discrete math/combinatorics there? (which means, anything from graph theory, optimization, enumeration, etc. can be possible topics) I just took a master course in Discrete Optimization and I thought of this subject as well. You could focus, fo...
Mon Nov 22, 2010 12:53 pm UTC
Forum: Mathematics
Topic: Some short questions about computational complexity theory
Replies: 16
Views: 1964

### Re: Some short questions about computational complexity theo

Ofcourse, instead of proving (ii) I could also show that KNAPSACK can be polynomially transferred to another NP-complete problem. You've got that around the wrong way. If you can show that a single NP-complete problem can be transformed into KNAPSACK polnomially, then you are good. Think about it, ...
Mon Nov 22, 2010 11:49 am UTC
Forum: Mathematics
Topic: Some short questions about computational complexity theory
Replies: 16
Views: 1964

### Re: Some short questions about computational complexity theo

I have a question about a homework exercise. In the exercise I'm supposed to prove that KNAPSACK is NP-complete. I thought that in order to do that I have to prove that (i) KNAPSACK is in NP and (ii) for all problems in NP I have to show that they transform polynomially to KNAPSACK. Ofcourse, instea...
Sun Nov 21, 2010 3:48 pm UTC
Forum: Mathematics
Topic: Some short questions about computational complexity theory
Replies: 16
Views: 1964

### Re: Some short questions about computational complexity theo

My last question probably made the least sense because I understand the issues related to that questions the least at the moment :) But thanks for your answers. Let's see if I get this. Suppose we have some problem X of which we know nothing about (that is, we know nothing about how to tackle the pr...
Sun Nov 21, 2010 1:40 pm UTC
Forum: Mathematics
Topic: Some short questions about computational complexity theory
Replies: 16
Views: 1964

### Some short questions about computational complexity theory

Hi all, I'm currently learning about complexity theory. I find it very interesting but I'm not entirely sure I grasp the differences between P and NP problems. I'd like to test myself by stating what I think I know about P and NP (and co-NP etc) and I hope someone more knowledgable than me can tell ...
Sun Oct 24, 2010 12:51 am UTC
Forum: Mathematics
Topic: Orthogonal curvilinear coordinate system
Replies: 5
Views: 1727

### Re: Orthogonal curvilinear coordinate system

I am trying to find a curvilinear coordinate system (i.e. basis vectors and transformation)

What does this mean exactly?
Wed Sep 22, 2010 3:05 pm UTC
Forum: Mathematics
Topic: Polynomial equivalence of two encodings of a graph
Replies: 3
Views: 1522

### Re: Polynomial equivalence of two encodings of a graph

Polynomial equivalency here means that you can transform one encoding into another in polynomial time, where the latter means the amount of operations you have to perform is a polynomial in terms of input lenght (here your |V| = n and |E| = m). If you have a certain algorithm that does something yo...
Wed Sep 22, 2010 2:18 pm UTC
Forum: Mathematics
Topic: Polynomial equivalence of two encodings of a graph
Replies: 3
Views: 1522

### Polynomial equivalence of two encodings of a graph

I've just started taking a course on discrete optimization. Because I have not yet had any dealing with graph theory I have some catching up to do. The first question I got in a homework set is the following: "Given an undirected graph G=(V,E) with |V|=n and |E|=m one can encode the graph using...
Fri Jun 11, 2010 8:39 am UTC
Forum: Mathematics
Topic: Infinite random walks
Replies: 6
Views: 1238

### Re: Infinite random walks

This was not written by a professor at my university, by the way.
Thu Jun 10, 2010 12:49 pm UTC
Forum: Mathematics
Topic: Infinite random walks
Replies: 6
Views: 1238

### Re: Infinite random walks

Ah yeah, I thought it was weird... Thanks for the help!w
Thu Jun 10, 2010 11:11 am UTC
Forum: Mathematics
Topic: Infinite random walks
Replies: 6
Views: 1238

### Infinite random walks

I've recently started reading about random walks. In this paper, there is a section which tries to find the probability of returning an infinite number of times to the origin when doing a random walk in 1-, 2- and 3-D. I don't understand the proof quite yet. If you look at the 1D and 2D case, you ca...
Wed Jun 02, 2010 11:34 am UTC
Forum: Mathematics
Topic: About an infinite sum of indicator functions
Replies: 4
Views: 2030

### Re: About an infinite sum of indicator functions

I guess it is equal to [imath]k[/imath]?
Wed Jun 02, 2010 10:42 am UTC
Forum: Mathematics
Topic: About an infinite sum of indicator functions
Replies: 4
Views: 2030

### About an infinite sum of indicator functions

I was reading about indicator functions the other day and I formulated the following question: Suppose that we're given a sequence of events \{A_n\}_{n=1}^{\infty} and we consider for every event A_k its indicator function I_k where I_k = 1 if A_k occurs and I_k = 0 if it doesn't. If we somehow can ...
Tue May 25, 2010 9:53 am UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

You might be right, but I don't have a very broad physics background, so no intuitive explainations for me :p
Mon May 24, 2010 1:20 pm UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

I have another question on an exercise in my book (not homework). Consider a smooth map f:M \rightarrow N where M = \mathbf{R}^m and N = \mathbf{R}^n . We consider the graph of f, which is given by G_f=\{(x,f(x)) \in M\times N | x \in M\} I now have to prove that for an arbritrary &#...
Fri May 21, 2010 12:55 pm UTC
Forum: Mathematics
Topic: help me with my date
Replies: 14
Views: 3435

### Re: help me with my data

Velifer wrote:
your sig wrote:there are three kinds of people in the world,man woman and mathematician.

Drop this one, go outside, find a woman.

I think he's way ahead of you on that one.
Fri May 21, 2010 7:12 am UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

I'm trying to learn analysis on manifolds as well and finding it quite hard. My main question now is one of intuition: Why are differential forms the correct objects to work with, as opposed to vector fields? What would go wrong if we only worked with vector fields? Munkres mentions a problem arisi...
Thu May 20, 2010 12:37 pm UTC
Forum: Science
Topic: Science and programming
Replies: 13
Views: 1878

### Re: Science and programming

I'm thinking about learning C++ for the same reasons the thread starter mentioned. I'm a math student and I've taken two courses on programming (introductory courses, really) in Java but I keep hearing about how C++ is used a lot in scientific programming. Is this true?
Wed May 19, 2010 7:47 pm UTC
Forum: Mathematics
Topic: problem with induction.
Replies: 8
Views: 1272

### Re: problem with induction.

No. Try reading such a proof again, for instance here: http://en.wikipedia.org/wiki/Mathematical_induction#Example . The point is that you don't assume your statement is true for all numbers n , but for only one. The point is that you eventually show that - The base case is true; - if your statement...
Tue May 18, 2010 2:08 pm UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

I would appreciate some input on how to solve this exercise (yes, it's homework): "Show that for every x \in \mathbf{S}^n \subset \mathbf{R}^{n+1} the tangent space T_x\mathbf{S}^n satisfies $$T_x\mathbf{S}^n = \{\xi \in \mathbf{R}^{n+1} | <\xi,x> = 0 \} = \mathbf{L}(\{x\})^{\perp}$$ where \mat...
Tue May 11, 2010 1:41 pm UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

Excellent read, thank you. I have some more questions later on though. I hope this will help others who are struggling with manifold theory.
Sat May 08, 2010 1:52 am UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

Thanks, I will take that to heart. I'm sure I'll be asking more questions later on. Let's hope that this thread helps others who are trying to study this subject in some way!
Thu May 06, 2010 2:11 pm UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

I think I have at least an intuitive grasp of what's going on now, thanks a lot guys :) I have another question after doing some exercises. I have to prove, given an manifold M in R^n, a subset N in R^p and a diffeomorphism f:M->N, that (1) N is a manifold and (2) dim(M) = dim(N). I don't think I ca...
Tue May 04, 2010 2:13 pm UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

What helped me grok this kind of thing is throwing away the idea of functions. Instead, think of objects that operate on other things. Have the operation occur by adjacency (to start with). So a function f is an object that operates on values in the domain. The result of the operation is an element...
Mon May 03, 2010 3:45 pm UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Re: Analysis on manifolds is gving me nightmares!

It is not natural because it "depends" on your choice of basis. What this means is that what functionals your isomorphism spits out depends on your choice of basis. Let's keep things concrete and work in R^2. Then one basis is the standard basis (1,0),(0,1). Then any vector v=a(1,0)+b(0,1...
Mon May 03, 2010 12:17 pm UTC
Forum: Mathematics
Topic: Analysis on manifolds is gving me nightmares!
Replies: 21
Views: 3043

### Analysis on manifolds is gving me nightmares!

Hi all, at the moment I'm working on the last course I have to take in order to get my bachelors degree. It is a course on analysis on manifolds. The only problem: I think I'm too retarded to understand the subject. It also doesn't help that the book our course uses isn't very well written. I do how...
Fri Apr 02, 2010 12:26 am UTC
Forum: General
Topic: It bothers me when a science theory is named after a person
Replies: 47
Views: 4581

### Re: It bothers me when a science theory is named after a per

What's wrong with Galois Theory?
Tue Mar 30, 2010 4:47 pm UTC
Forum: Coding
Topic: Smart ways to solve an equation in C++
Replies: 13
Views: 1844

### Re: Smart ways to solve an equation in C++

No that's my fault. I made a typo; it should be 1/6 +- 1/3sqrt(24m-1)

@ phlip: so what I could do is, if x = 1/6 +- 1/3sqrt(24m-1), check whether x(3x-1) % 2 == 0 right? But then my question remains; of what type should x be?
Tue Mar 30, 2010 12:46 am UTC
Forum: Coding
Topic: Smart ways to solve an equation in C++
Replies: 13
Views: 1844

### Re: Smart ways to solve an equation in C++

.. I can't believe I didn't realize that. This is my problem with programming; most of the time I'm so concerned with how I am going to design my program that I fail to see the most obvious solutions (programming courses at my university used to give me nightmares). Thanks for the hint! One of the ...
Mon Mar 29, 2010 4:22 pm UTC
Forum: Coding
Topic: Smart ways to solve an equation in C++
Replies: 13
Views: 1844

### Re: Smart ways to solve an equation in C++

.. I can't believe I didn't realize that. This is my problem with programming; most of the time I'm so concerned with how I am going to design my program that I fail to see the most obvious solutions (programming courses at my university used to give me nightmares). Thanks for the hint! But now I am...
Mon Mar 29, 2010 3:46 pm UTC
Forum: Coding
Topic: Making a game
Replies: 6
Views: 1105

### Re: Making a game

dvangel,

I'm very curious to see the source code of your game. I've recently started to learn C++ and I'd love to be able to create a platform game from scratch. I think I might learn a lot by looking at your code. Would it be okay if I looked at it?
Mon Mar 29, 2010 3:41 pm UTC
Forum: Coding
Topic: Smart ways to solve an equation in C++
Replies: 13
Views: 1844

### Smart ways to solve an equation in C++

I've decided to learn C++ a couple of days ago. To help me learn the language I've started to do a couple of Project Euler problems in C++. At the moment I'm working at problem 44 ( http://projecteuler.net/index.php?section=problems&id=44 ). As part of the solution I have to write a function tha...