## Search found 145 matches

- Sat Feb 10, 2018 11:33 pm UTC
- Forum: Science
- Topic: Squat Physics
- Replies:
**3** - Views:
**2538**

### Re: Squat Physics

If you support the bar with your arms, then that weight is necessarily applied through your shoulders. Your shoulders are probably higher up your back than the bar would normally be, which means that the force exerted by your legs/hips has to be greater. That said, I think the primary issues with su...

- Wed Jan 17, 2018 3:56 am UTC
- Forum: Fictional Science
- Topic: Most realistic magic system and areas for improvement
- Replies:
**27** - Views:
**9268**

### Re: Most realistic magic system and areas for improvement

I haven't read this in a long time, but I really enjoyed the magic system from Rick Cook's "The Wiz Biz". The main character is a modern day (well, in 1989) programmer who is summoned to a fantasy world, so he starts programming magic by making tiny little spells that are the equivalent of...

- Wed Dec 27, 2017 10:23 pm UTC
- Forum: Science
- Topic: Melting Wood
- Replies:
**18** - Views:
**4028**

### Re: Melting Wood

What are other compounds that cannot melt under normal conditions and can any of them melt under abnormal conditions? This might be a little (a lot?) more pedantic than you were looking for, but many substances cannot melt under standard conditions (i.e., room temperature, atmospheric pressure, etc...

- Wed Dec 27, 2017 9:36 am UTC
- Forum: Science
- Topic: Melting Wood
- Replies:
**18** - Views:
**4028**

### Re: Melting Wood

What are other compounds that cannot melt under normal conditions and can any of them melt under abnormal conditions? This might be a little (a lot?) more pedantic than you were looking for, but many substances cannot melt under standard conditions (i.e., room temperature, atmospheric pressure, etc...

- Thu Dec 14, 2017 4:16 pm UTC
- Forum: Mathematics
- Topic: Bump Function
- Replies:
**7** - Views:
**4234**

### Re: Analytic Bump Function

It’s also worth noting that your title doesn’t match your question, which is good because the title is impossible. A bump function can be infinitely differentiate, but it cannot be analytic.

- Tue Nov 21, 2017 8:08 pm UTC
- Forum: Mathematics
- Topic: Assassin Problem over the Internet
- Replies:
**40** - Views:
**8385**

### Re: Assassin Problem over the Internet

Again, the fact that people can completely sabotage the game if they want to isn't a weakness of any particular method of assigning roles, because someone could refuse to play in any system. There’s some difference, because “refusing to play” is public, but in this case you can sabotage the game pr...

- Mon Nov 20, 2017 10:39 pm UTC
- Forum: Mathematics
- Topic: Assassin Problem over the Internet
- Replies:
**40** - Views:
**8385**

### Re: Assassin Problem over the Internet

If Bob wants to target Alice, he can wait to see who his target is before sending his own email. If someone other than Alice is his target, he then sends an email to the wrong person, which will cause the game to fail. He could keep doing this until he gets Alice as a target. I think an obvious solu...

- Wed Nov 01, 2017 9:14 pm UTC
- Forum: Coding
- Topic: code for melting computer
- Replies:
**7** - Views:
**6881**

### Re: code for melting computer

Even on fairly modern computers, it’s possible to cause a lot of damage. For instance, you can brick your battery (or possibly set it on fire) or set your printer on fire . On older machines, I imagine it’s much easier to do terrible things. This stackoverflow thread mentions melting magnetic-core m...

- Wed Nov 01, 2017 4:56 am UTC
- Forum: Science
- Topic: Is meditation just relaxation?
- Replies:
**15** - Views:
**2725**

### Re: Is meditation just relaxation?

I'm curious how people with a mainstream scientific view approach this question. Sam Harris seems like a relevant example. I'm not nearly familiar enough with his arguments (or meditation in general) to explain his viewpoint or really pass judgement on it, but he is a neuroscientist who believes th...

- Fri Oct 07, 2016 11:27 pm UTC
- Forum: Mathematics
- Topic: Why does x=y -> f(x)=f(y)?
- Replies:
**12** - Views:
**3450**

### Re: Why does x=y -> f(x)=f(y)?

Sure, I wasn't being very precise about my definitions of rational numbers, but my point was that such an f is not a function under any standard definitions of the terms. (In fact, I remember this being one of the textbook examples in my undergrad "introduction to proofs" class.) Admittedl...

- Fri Oct 07, 2016 9:01 pm UTC
- Forum: Mathematics
- Topic: Why does x=y -> f(x)=f(y)?
- Replies:
**12** - Views:
**3450**

### Re: Why does x=y -> f(x)=f(y)?

And how you define =. If you say x=y but f(x) != f(y), then there must be something not-quite-x about y. Which generally is not a notion supported by =. Try ~? A little of both, I suppose, but this feels more like a property of functions than a property of equality to me. The first example that com...

- Tue Oct 04, 2016 9:12 pm UTC
- Forum: Mathematics
- Topic: Factorization using modular arithmetic
- Replies:
**41** - Views:
**10491**

### Re: Factorization using modular arithmetic

Sizik wrote:Are you looking for something with more restrictions than x = 1, y = A - 1?

Or x = 0, y = A. Now that we have two solutions, factoring should be easy.

- Sat Sep 24, 2016 9:53 pm UTC
- Forum: Mathematics
- Topic: Dataset of linear systems
- Replies:
**21** - Views:
**4442**

### Re: Dataset of linear systems

All of those commands work with sparse matrices. Some may be much slower than others, but that's because some require intermediate steps with many nonzero elements. For instance, it is difficult to calculate the 2-norm of a big matrix even if it is sparse. That's not a Matlab problem, it's just a ma...

- Sat Sep 24, 2016 6:18 pm UTC
- Forum: Mathematics
- Topic: Dataset of linear systems
- Replies:
**21** - Views:
**4442**

### Re: Dataset of linear systems

https://www.mathworks.com/help/matlab/linear-algebra.html You can use Matlab to calculate pretty much anything you're interested in -- most of it with only a line or two of code. Are you having any trouble understanding the Matlab commands? The properties you mentioned (size, norm, trace, e.g.) hav...

- Wed Sep 21, 2016 10:29 pm UTC
- Forum: Mathematics
- Topic: Dataset of linear systems
- Replies:
**21** - Views:
**4442**

### Re: Dataset of linear systems

1) On my laptop (which is nothing special) and with a 2012 version of Matlab I can solve 10,000 x 10,000 dense systems in ~20 secons, but I can't store a 100,000 x 100,000 dense matrix. (I believe this is because Matlab insists on using contiguous blocks of memory for matrices. Sparse systems can be...

- Wed Sep 21, 2016 5:06 am UTC
- Forum: Mathematics
- Topic: Dataset of linear systems
- Replies:
**21** - Views:
**4442**

### Re: Dataset of linear systems

Alright. Matlab should be fine for your purposes. It has several builtin solvers/matrix factorization routines for things like LU decomposition, QR decomposition, Cholesky factorization, GMRES and PCG. You can solve systems with these commands in a few lines of code. You specifically mentioned Jacob...

- Tue Sep 20, 2016 7:46 pm UTC
- Forum: Mathematics
- Topic: Dataset of linear systems
- Replies:
**21** - Views:
**4442**

### Re: Dataset of linear systems

1) I still maintain that you should use random matrices. Some solvers (e.g. Gaussian elimination or QR factorization) work on almost every (in the technical sense) matrix, so you can just randomly generate each element. Other solvers (e.g. the Jacobi method or conjugate gradient) need a special stru...

- Mon Sep 19, 2016 11:46 pm UTC
- Forum: Mathematics
- Topic: Dataset of linear systems
- Replies:
**21** - Views:
**4442**

### Re: Dataset of linear systems

Thank you for the answers and sorry for the delay, I had a health problem. I want to find an already existed dataset because i want my results to be comparable with the results of other people. Please help me, it is for my postgraduate work I don't understand. You mentioned in the first post that y...

- Mon Jul 25, 2016 4:13 am UTC
- Forum: Mathematics
- Topic: Dataset of linear systems
- Replies:
**21** - Views:
**4442**

### Re: Dataset of linear systems

If you want a matrix of a particular rank, you could randomly generate the SVD. This seems to work:

Code: Select all

`[U,R] = qr(rand(m,m)); `

[V,R] = qr(rand(n,n));

S = zeros(m,n);

for i = 1:r

S(i,i) = rand(1)

end

A = U*S*V'

- Mon Jul 11, 2016 11:39 pm UTC
- Forum: Science
- Topic: The Solar System on a square-root scale
- Replies:
**17** - Views:
**4938**

### Re: The Solar System on a square-root scale

This is only vaguely on topic, but I'm excited to have an excuse to share the link. My high school was near the sun end of a linear scale model of the solar system. The sun is 15m in diameter and pluto is about 2.5cm in diameter and 65km away. Clearly linear scales are the best.

- Fri Jun 03, 2016 8:21 am UTC
- Forum: Mathematics
- Topic: Math: Fleeting Thoughts
- Replies:
**408** - Views:
**132597**

### Re: Math: Fleeting Thoughts

Eebster the Great wrote:By the way, MostlyHarmless, that link is only accessible to U Washington students.

Hmm, I'm not sure how that happened. I was off campus when I found it, but the link won't work for me anymore either. This one should work better.

- Fri Jun 03, 2016 12:04 am UTC
- Forum: Mathematics
- Topic: Math: Fleeting Thoughts
- Replies:
**408** - Views:
**132597**

### Re: Math: Fleeting Thoughts

Eebster the Great wrote:And every non-discrete, nontrivial topological space also has subsets which are neither closed nor open.

Spaces with no sets that are neither closed nor open (that's a mouthful) are called door spaces. Cauchy described one type, and the link gives a complete classification.

- Tue May 31, 2016 9:54 pm UTC
- Forum: Mathematics
- Topic: Math: Fleeting Thoughts
- Replies:
**408** - Views:
**132597**

### Re: Math: Fleeting Thoughts

There is some third possibility. For example, the set [0,1) is neither open nor closed in the real line with the standard topology.

- Tue Sep 29, 2015 8:08 pm UTC
- Forum: Mathematics
- Topic: Infinite-Dimensional Geometry
- Replies:
**0** - Views:
**2408**

### Infinite-Dimensional Geometry

I'm working with an infinite-dimensional affine space, and I want to do some geometry with it. It seems like the standard approach is to choose a finite-dimensional "submanifold" where everything works nicely, but I can't find any ways of choosing these submanifolds that don't feel extreme...

- Thu Sep 03, 2015 4:49 pm UTC
- Forum: Coding
- Topic: Sanitizing Code Submissions
- Replies:
**6** - Views:
**3657**

### Re: Sanitizing Code Submissions

Thanks for the suggestions! I've checked out isolate, plus some of the other suggestions from that stack exchange thread and there are some good ideas there. I had considered the virtual machine approach before, but other threads have validated my fears that it would be too slow for this sort of thi...

- Sun Aug 30, 2015 7:23 am UTC
- Forum: Coding
- Topic: Sanitizing Code Submissions
- Replies:
**6** - Views:
**3657**

### Sanitizing Code Submissions

I'm trying to set up a homework submission system for my class (introduction to numerical methods). The students will submit a python file, then I'll run it and check the output. I realized that someone could easily put some malicious code in one of their homework files and cause serious problems. H...

- Wed Jul 29, 2015 11:14 pm UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**9907** - Views:
**1858120**

### Re: Coding: Fleeting Thoughts

I suspect this is mainly because the output needs to be a cell if it contains different types. If the output is a cell (or potentially worse, if it may or may not be a cell) then it's very easy to get an error by using resCell{1} instead of resCell(1) or vice versa. Wait a minute, are you telling m...

- Wed Jul 29, 2015 8:53 pm UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**9907** - Views:
**1858120**

### Re: Coding: Fleeting Thoughts

@Flumble: I'm not entirely sure what you're asking, but I think you're looking for resCell = arrayfun(funcPtr, argList..., 'UniformOutput', False) (Alternatively, you could try cellfun .) Thanks. I'm a little confused as to why matlab would be more restrictive by default, but it can't be helped I g...

- Wed Jul 29, 2015 1:04 am UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**9907** - Views:
**1858120**

### Re: Coding: Fleeting Thoughts

@Flumble: I'm not entirely sure what you're asking, but I think you're looking for resCell = arrayfun(funcPtr, argList..., 'UniformOutput', False) (Alternatively, you could try cellfun .) As a side note, why do you hate anonymous functions? I can understand trying not to confuse students (I teach an...

- Thu Jul 16, 2015 11:33 pm UTC
- Forum: Coding
- Topic: Python - Best way to turn string of digits into list of ints
- Replies:
**11** - Views:
**4319**

### Re: Python - Best way to turn string of digits into list of

Thanks for the replies! I picked up "Learning Python" by Mark Lutz and he seems to mostly agree: list comprehensions are usually better than generators unless you have a very large number of digits. (I've been solving some project Euler problems, and things like "the first ten million...

- Wed Jul 15, 2015 6:43 am UTC
- Forum: Coding
- Topic: Python - Best way to turn string of digits into list of ints
- Replies:
**11** - Views:
**4319**

### Python - Best way to turn string of digits into list of ints

First off, I apologize if this belongs in another thread. The best I could find was this , but I didn't want to necro a 6 year old thread just to post something almost on-topic. On to the question: I've been teaching myself python (3.4), mostly to use for applied math stuff. In a lot of problems I'v...

- Sat Jul 11, 2015 10:07 am UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**9907** - Views:
**1858120**

### Re: Coding: Fleeting Thoughts

Today I learned that closures are weird. Maybe tomorrow I will learn why they're useful.

- Sun Mar 15, 2015 6:19 am UTC
- Forum: Science
- Topic: Criticality of Thermodynamic Systems
- Replies:
**2** - Views:
**1910**

### Re: Criticality of Thermodynamic Systems

First, thanks for your reply. I am fairly familiar with P-T phase diagrams, but I am not entirely clear how to apply them to my problem. For reference, I am studying systems discussed in this paper: http://link.springer.com/article/10.1007/s10955-011-0229-4 If that's behind a paywall for you, I'll s...

- Mon Mar 09, 2015 11:42 pm UTC
- Forum: Science
- Topic: Criticality of Thermodynamic Systems
- Replies:
**2** - Views:
**1910**

### Criticality of Thermodynamic Systems

Hi everyone, I'm doing some research in applied math (modelling with large datasets), and I've been drawn into the realm of statistical thermodynamics. Unfortunately, I am not a physicist. (I somehow managed to get a bachelor's and masters in math applied math without taking a physics class.) This m...

- Sun Feb 15, 2015 11:49 pm UTC
- Forum: Mathematics
- Topic: Math: Fleeting Thoughts
- Replies:
**408** - Views:
**132597**

### Re: Math: Fleeting Thoughts

The maximum possible temperature of a thermodynamic system is the limit-supremum as energy goes to infinity of energy divided by volume entropy. If you've got tex: T_{max} = \limsup_{E\to\infty}\frac{E}{\ln V(E)}. Maybe that sounds like it should go in the science forum, but it mostly comes from som...

- Tue Dec 16, 2014 6:03 am UTC
- Forum: Mathematics
- Topic: Math: Fleeting Thoughts
- Replies:
**408** - Views:
**132597**

### Re: Math: Fleeting Thoughts

I think you're talking about a Bernoulli flow. They're covered by Ornstein's Theorem.

- Mon Aug 04, 2014 12:30 am UTC
- Forum: Mathematics
- Topic: A nonlinear second-order ODE
- Replies:
**10** - Views:
**3614**

### Re: A nonlinear second-order ODE

You seem to have missed something in the right hand side. I can't get rid of the \int y term with that method, and mathematica can solve the first order equation you gave (and it's not a solution to the original). I'm not really sure how to make any headway on the problem, though. cyanyoshi's sugges...

- Thu Jul 31, 2014 6:25 pm UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**9907** - Views:
**1858120**

### Re: Coding: Fleeting Thoughts

I'm very proud of myself this morning. I made a python script to download a few hundred zip files, extract them somewhere, then delete the zip files. Not much, I know, but this is my first working python code and the first time in about a decade that I've coded anything besides numerical stuff. Even...

- Sat Jun 28, 2014 7:12 am UTC
- Forum: Mathematics
- Topic: The Most Important Math
- Replies:
**48** - Views:
**18871**

### Re: The Most Important Math

Meh, dynamical systems are clearly the most important. Bayesian updates are just a dynamical system on probability distributions.

- Fri Jan 24, 2014 2:51 am UTC
- Forum: Mathematics
- Topic: Matlab alternatives
- Replies:
**4** - Views:
**2139**

### Re: Matlab alternatives

Mmm, if you actually want "Matlab for free", then Octave is a great choice. The real question is why you want Matlab instead of something like Python, C or Fortran. I still use Matlab for school sometimes, but in my experience no one does any serious numerics in it. What are you trying to ...