## Search found 532 matches

- Wed Jul 29, 2015 6:52 am UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**10016** - Views:
**2034507**

### Re: Coding: Fleeting Thoughts

Lastly, it took me a day to find out matlab does have a function to get the magnitude of a vector: norm . It was neither length nor size and magnitude doesn't exist at all. Nor do normalize or unit exist for normalizing a vector. :cry: So, it used to mystify me why matlab doesn't have a normalize f...

- Sat Mar 14, 2015 9:31 pm UTC
- Forum: Mathematics
- Topic: Graph Theory definition question
- Replies:
**6** - Views:
**1920**

### Re: Graph Theory definition question

So, your worry about slightly-non-standard names for the concept is correct -- this concept is usually called the treewidth of a graph: http://en.wikipedia.org/wiki/Treewidth . It turns out that there are several equivalent definitions, and the unfortunate thing is that none of them is particularly ...

- Sat Jan 17, 2015 5:38 am UTC
- Forum: Science
- Topic: "connecting" the earth and the moon
- Replies:
**24** - Views:
**6163**

### Re: "connecting" the earth and the moon

Pfah! Moving the moon is too hard. Let's instead slow down the earth so that the moon's current orbit is now geosynchronous. Nobody will notice the difference, right?

- Fri Dec 26, 2014 4:11 am UTC
- Forum: Computer Science
- Topic: how to communicate under heavy targeted surveillance
- Replies:
**13** - Views:
**8587**

### Re: how to communicate under heavy targeted surveillance

You can set up quantum cryptosystems that cannot be eavesdropped on without being detected. You still have the issue of key exchange though Wrong! Quantum crypto has no key exchange issue. What you do need to set up is a qubit transfer channel, though. Once you have that, you can exchange a key in ...

- Thu Nov 27, 2014 5:28 pm UTC
- Forum: Mathematics
- Topic: Clarification on definition of category
- Replies:
**6** - Views:
**2637**

### Re: Clarification on definition of category

Sorry, I did misspeak. Let's call this putative "category" C (it's not actually a category, but that's what we're trying to show). It has only one object A. So when I said "identity element" I meant "the identity map for the only object". There are lots of possible ways...

- Thu Nov 27, 2014 4:09 pm UTC
- Forum: Mathematics
- Topic: Clarification on definition of category
- Replies:
**6** - Views:
**2637**

### Re: Clarification on definition of category

Another concrete example: You can think of any group as a category with only one "object". Note that this is not the same as considering the category of groups, Group. We're just looking at one group, maybe let's pick a very simple one, Z_2. How can we make this a category? Well, we have o...

- Sun Nov 09, 2014 11:40 pm UTC
- Forum: Computer Science
- Topic: Choosing the best quiz team
- Replies:
**8** - Views:
**4397**

### Re: Choosing the best quiz team

Covering problem sounds about right. You have n people, which gives you n different sets S, where S_i is the set of questions that person i got correct. You want to pick 3 of these sets to cover as many questions as possible. Set cover is NP-hard so there isn't a general algorithm to solve it, but t...

- Sun Nov 09, 2014 11:30 pm UTC
- Forum: Science
- Topic: Can protons be extracted from an acid solution?
- Replies:
**26** - Views:
**5593**

### Re: Can protons be extracted from an acid solution?

If you had all the protons from 1L of water in a sphere, you'd have about 53MC of charge in a sphere 6.2cm in radius, for a total electrostatic potential energy of 2.4e28 J, or 6 trillion megatons of TNT. For comparison, that's only 4 orders of magnitude less than the gravitational binding energy o...

- Thu Oct 23, 2014 4:57 am UTC
- Forum: Mathematics
- Topic: How many ways can you prove x + 1/x >= 2?
- Replies:
**23** - Views:
**16781**

### Re: How many ways can you prove x + 1/x >= 2?

x + 1/x is convex. The derivative at x = 1 is 1 - 1 = 0, and 1 + 1/1 = 2, so since convex curves lie above any tangent line, x + 1/x >= 2. Have been doing lots of convex optimization recently, so was my first thought.

Pretty sure that counts as different than all prior proofs.

Pretty sure that counts as different than all prior proofs.

- Tue Jul 22, 2014 1:36 am UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**10016** - Views:
**2034507**

### Re: Coding: Fleeting Thoughts

For keeping packages synchronized between machines, have you considered Vundle ? That should reduce your problem to keep your .vimrc synchronized between machines (since packages then are specified in your .vimrc with vundle). I haven't actually tried using it across machines, but I do use it for pa...

- Wed Jul 16, 2014 6:28 pm UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**10016** - Views:
**2034507**

### Re: Coding: Fleeting Thoughts

Ahh, got it. So, not in-place like an in-place sort. The extra allocated memory ends up being 2x the amount of memory you started with anyways, which ends up being not bad.

- Wed Jul 16, 2014 3:24 pm UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**10016** - Views:
**2034507**

### Re: Coding: Fleeting Thoughts

So, the code claims to be an in-place FFT, but from looking at the documentation for valarray, the lines: CArray even = x[std::slice(0, N/2, 2)]; CArray odd = x[std::slice(1, N/2, 2)]; seem to create a copy of the data? Maybe I just misunderstand valarray and slices, though.

- Sat Mar 08, 2014 9:47 am UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**10016** - Views:
**2034507**

### Re: Coding: Fleeting Thoughts

ahammel wrote:Bladow! New keyboard

My! That is a very *vim* keyboard!

- Sat Mar 01, 2014 7:21 pm UTC
- Forum: Fictional Science
- Topic: How close can two planets' surfaces get?
- Replies:
**15** - Views:
**11189**

### Re: How close can two planets' surfaces get?

Well yes, but the problem with the 6-foot distance is really that two realistic planets would become too-tidally-locked, in that tidal forces would rip them apart.

- Sat Mar 01, 2014 6:36 pm UTC
- Forum: Fictional Science
- Topic: How close can two planets' surfaces get?
- Replies:
**15** - Views:
**11189**

### Re: How close can two planets' surfaces get?

speising wrote:and they certainly couldn't be both tidally locked to each other. for that to work, they'd need to be in geosynchronous orbits.

Actually, Pluto and Charon are both tidally locked to each other. So that on its own isn't impossible.

- Sat Mar 01, 2014 5:37 pm UTC
- Forum: Fictional Science
- Topic: How close can two planets' surfaces get?
- Replies:
**15** - Views:
**11189**

### Re: How close can two planets' surfaces get?

Because of the Roche Limit, if the planets are each held together by their own self-gravity, they cannot be this close together. As for rigid planets, I suspect you'd need unobtanium, but I'm not sure how to do the math on that.

- Tue Feb 25, 2014 4:26 pm UTC
- Forum: Mathematics
- Topic: Generality of polynomial approximation
- Replies:
**7** - Views:
**2512**

### Re: Generality of polynomial approximation

EDIT: I just realized a problem with my counter-example. f(0) = g(0), but the OP stated f(x) > g(x) on the interval. It may be possible to salvage this, but I'm not sure. The mutual restrictions of min(f-g) does not exist, f > g, and f and g continuous on a close interval are difficult to hit I thi...

- Tue Feb 18, 2014 3:32 pm UTC
- Forum: Science
- Topic: Boiling in the greenhouse
- Replies:
**19** - Views:
**4119**

### Re: Boiling in the greenhouse

Yeah, if you re-condense the water, then you get a solar still, which is a pretty handy piece of equipment to have if you're stuck in the wilderness.

- Tue Feb 11, 2014 4:11 pm UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**10016** - Views:
**2034507**

### Re: Coding: Fleeting Thoughts

\Why is this allowed? What is the point of const if you can just cast it away like that? Because C-style casts regularly break the type system and you shouldn't use them? There's also const_cast, which does the same thing, but is easier to grep for and specifically alerts anyone reading the code to...

- Tue Feb 11, 2014 6:22 am UTC
- Forum: Mathematics
- Topic: almost homework, a system in 3 unknowns
- Replies:
**11** - Views:
**3254**

### Re: almost homework, a system in 3 unknowns

But it's not rational and some luck is needed to find it Or, like, use the cubic formula . Or find it numerically by, e.g., Newton's method. Not sure what "luck" has to do with it, unless you're picking real numbers randomly and seeing if they're roots. If so, "good luck" with t...

- Mon Feb 03, 2014 3:18 pm UTC
- Forum: Mathematics
- Topic: How should I approach this optimization problem
- Replies:
**16** - Views:
**3318**

### Re: How should I approach this optimization problem

Yup, sorry about the -2 lambda, that was a mistake. Anyways, Lagrange multipliers are great for turning constrained optimization problems into unconstrained ones. This ends up being most useful in situations where unconstrained optimization is easy (i.e., where you can apply calculus without too muc...

- Mon Feb 03, 2014 5:14 am UTC
- Forum: Mathematics
- Topic: How should I approach this optimization problem
- Replies:
**16** - Views:
**3318**

### Re: How should I approach this optimization problem

Ok, so the first thing to know how to do is unconstrained optimization: For one variable, you probably already know that the global optimum is going to either occur at a critical point (a place where the derivative is equal to zero) or at +/- infinity. So the way you optimize a function of one varia...

- Sun Jan 26, 2014 11:09 pm UTC
- Forum: Mathematics
- Topic: Need help understanding the usual topology on R
- Replies:
**23** - Views:
**5771**

### Re: Need help understanding the usual topology on R

That is the "definitional if" (possibly not the real word for it), which is actually an if and only if. So, both the things you just said have to hold. It's a bit of a quirk of mathematical prose that we often say "We define x to be a foo if bar happens" which really means x is a...

- Sat Jan 25, 2014 8:22 am UTC
- Forum: Science
- Topic: Vacuum particles and gravity
- Replies:
**6** - Views:
**2287**

### Re: Vacuum particles and gravity

Energy density also exerts gravity in General Relativity, so there's no difference between a certain amount of background energy (in the form of, e.g. photons) vs. a virtual particle pair, in terms of the gravity it exerts. For your second question, the virtual particle pairs that we usually talk ab...

- Fri Nov 01, 2013 6:35 am UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**10016** - Views:
**2034507**

### Re: Coding: Fleeting Thoughts

Is it wrong that I wrote my first set of real unit tests the other day, whilst completing a pre-interview programming screen? Is it wrong that the first time I actually worked with any code that utilised inheritance was at an interview? I think you're doing well. I didn't do either of those things ...

- Mon Oct 28, 2013 11:31 pm UTC
- Forum: Fictional Science
- Topic: Dual world engineering
- Replies:
**33** - Views:
**9956**

### Re: Dual world engineering

Huh. Somewhere, I'd picked up the intuition that if the effective gravity (actual gravity + centrifugal force) was higher in one location and lower in another, then mass would move around so that they were the same both places. But it occurs to me that that's an argument by way of "getting for...

- Mon Oct 28, 2013 5:37 pm UTC
- Forum: Fictional Science
- Topic: Dual world engineering
- Replies:
**33** - Views:
**9956**

### Re: Dual world engineering

No, the deformation actually exaggerates the effect slightly. Huh. Somewhere, I'd picked up the intuition that if the effective gravity (actual gravity + centrifugal force) was higher in one location and lower in another, then mass would move around so that they were the same both places. But it oc...

- Sun Oct 27, 2013 8:51 pm UTC
- Forum: Fictional Science
- Topic: Dual world engineering
- Replies:
**33** - Views:
**9956**

### Re: Dual world engineering

That's not how tidal forces work. Remember you are in freefall around the other planet. You are not heavier on the back side of the planet, because of the pull of the other planet. You are lighter because its pull is less and your orbit radius is greater. You would be the heaviest ninety degrees fr...

- Sat Oct 26, 2013 1:22 am UTC
- Forum: Mathematics
- Topic: n sided polygon, n < 3
- Replies:
**35** - Views:
**6791**

### Re: n sided polygon, n < 3

mike-l wrote:gmalivuk wrote:When did "poly" stop meaning "many"?

Polynomials anyone?

You mean the thing which is a sum of many monomials?

- Sat Oct 12, 2013 2:27 am UTC
- Forum: Mathematics
- Topic: Is this ambiguous? Is it fallacious?
- Replies:
**92** - Views:
**19393**

### Re: Is this ambiguous? Is it fallacious?

That's exactly what I mean by isomorphic, and you've got me dead to rights. Dunno how I missed that one. Perhaps we better rename mine the "Schrollini minimal commuting Z2 x Z2 faithful action group," or the "SMCZ2xZ2FAG" for short. They're both Z2xZ2 faithful group actions, so ...

- Fri Oct 11, 2013 11:09 pm UTC
- Forum: Mathematics
- Topic: Is this ambiguous? Is it fallacious?
- Replies:
**92** - Views:
**19393**

### Re: Is this ambiguous? Is it fallacious?

By the way, your definition of a system <S, X, Y> where X,Y are distinct involutions on S is known in Algebra as a " Group action ". More specifically, the functions {identity, X, Y, XY} are a group (they're the group Z_2 x Z_2), and a group action is a homomorphism G -> Perm(S), where Per...

- Wed Oct 09, 2013 3:59 am UTC
- Forum: Mathematics
- Topic: Is this ambiguous? Is it fallacious?
- Replies:
**92** - Views:
**19393**

### Re: Is this ambiguous? Is it fallacious?

Mathematically a binary system can be specified by a non-trivial instance of a function X where X(X(s 1 )) = s 1 . In such a system X(s 1 ) = s 2 , X(s 2 ) = s 1 . That is, when the operator is such that X(X(s 1 )) = s 1 the operator relates a pair of states {s 1 , s 2 } by providing a function tha...

- Wed Sep 18, 2013 1:53 pm UTC
- Forum: Science
- Topic: Earth as a column
- Replies:
**56** - Views:
**13613**

### Re: Earth as a column

Define an imaginary sphere with Earth's radius at the center of gravity of your object. If your object is completely contained within that sphere, it will have the same gravity as if it was uniformly distributed, regardless of what it actually looks like. This is only true if you end up with a sphe...

- Fri Sep 13, 2013 4:00 am UTC
- Forum: Science
- Topic: Could a dryer randomly fold clothing? (ongoing debate)
- Replies:
**13** - Views:
**8875**

### Re: Could a dryer randomly fold clothing? (ongoing debate)

So, this actually just happened to me! Or rather, one shirt out of an entire load ended up correctly folded (the rest of the load was the usual jumble). This is probably largely confirmation bias because I was already primed to look for folded laundry after the dryer, but it puts an upper bound for ...

- Wed Aug 21, 2013 9:09 am UTC
- Forum: Science
- Topic: There is only one* model consistent with CPT Symmetry
- Replies:
**129** - Views:
**39645**

### Re: There is only one* model consistent with CPT Symmetry

Man, I really thought this thread would be done by now. But, since it isn't, here's my complete, consistent axiomatization of a universe which is CPT symmetric which is clearly not our own: 1) There is nothing. 2) Nothing ever happens. 3) There are no charges (see 1 and 2...) This axiomatization is ...

- Sat Aug 10, 2013 8:45 am UTC
- Forum: Science
- Topic: There is only one* model consistent with CPT Symmetry
- Replies:
**129** - Views:
**39645**

### Re: There is only one* model consistent with CPT Symmetry

The following seems to be the meat of your argument: Objects that are constrained to only 2 possible states are binary objects. If we represent a binary object as b then a group of 3 binary objects can be represented as {b, b, b}. Given that all binary objects are equivalent then there is only one d...

- Tue Aug 06, 2013 8:11 am UTC
- Forum: Computer Science
- Topic: How prone are plagiarism-detection tools to false-positives?
- Replies:
**7** - Views:
**9462**

### Re: How prone are plagiarism-detection tools to false-positi

Having been on the using-end of these tools, my impression is that they generally allow a fairly high false-positive rate for a low false-negative rate, so that the tool can report all plausible cases of plagarism to the instructor. Then, it's up to the human to decide if the (much smaller pool of) ...

- Sun Jul 14, 2013 9:40 am UTC
- Forum: Mathematics
- Topic: Calculating a progressively more complicated battle
- Replies:
**6** - Views:
**1962**

### Re: Calculating a progressively more complicated battle

Honestly just doing lots of simulated, repeated trials is going to give you the most accurate answers with the least effort. Many of these problems can probably be worked out by hand, but the time to do so is much greater than just writing a few lines of code and running 10 million trials. Though, s...

- Fri Jun 28, 2013 4:48 am UTC
- Forum: Mathematics
- Topic: Calculus Question (Existence of a Function)
- Replies:
**7** - Views:
**3102**

### Re: Calculus Question (Existence of a Function)

What do you mean by: (exponential functions don't count as f(x) doesn't span R in that case) Do you mean that e^x doesn't cut it because the limit as x goes to -inf is 0? If that's your only problem, how can you modify e^x to get something that goes to -inf, but still looks basically the same as far...

- Sun Jun 23, 2013 10:50 am UTC
- Forum: Computer Science
- Topic: Building a foundation for CS/machine learning
- Replies:
**4** - Views:
**5777**

### Re: Building a foundation for CS/machine learning

Well, "not directly relevant to CS and machine learning" isn't strictly true. However, if you remember the following fact, you'll know basically everything Calculus has to say about machine learning (and... really this is pretty much the only part of Calculus that any CS ever uses): If you...