## Search found 263 matches

Fri May 20, 2011 7:50 pm UTC
Forum: Mathematics
Topic: Definition of concave function
Replies: 7
Views: 1083

### Re: Definition of concave function

skeptical scientist wrote:
Cleverbeans wrote:I believe you may have meant to say -|x| is concave or that |x| is convex, in either case your point is clearly valid and is one of the same mistake I made when first trying to tackle this problem.

Yes.

Govalent, did you figure this out?

Yeah, it's done. Thanks!
Tue May 17, 2011 8:55 pm UTC
Forum: Mathematics
Topic: Definition of concave function
Replies: 7
Views: 1083

### Re: Definition of concave function

"Thinks the same as you do" = glosses over important details? I should think not. If you want to prove something, it is important to get the details right. P.S. I added the following sentence to my previous post, but you had already replied: "If you're stuck on the proof, one directi...
Tue May 17, 2011 8:53 pm UTC
Forum: Mathematics
Topic: Definition of concave function
Replies: 7
Views: 1083

### Re: Definition of concave function

Well, that statement is not actually true. For one thing, the definition you gave for concave rather than strictly concave should intuitively be equivalent to f''(x)≥0, not f''(x)>0. Even still, you run into trouble with functions like |x|, which are concave without being differentiable. So start b...
Tue May 17, 2011 8:12 pm UTC
Forum: Mathematics
Topic: Definition of concave function
Replies: 7
Views: 1083

### Definition of concave function

So last year in my calculus course we used this definition of concavity: Let f be defined on [a,b]. We call f concave iff f(a + t(b-a)) \geq f(a) + t(f(b) - f(a)) \quad \forall t \in (0,1) Now I want to prove that, for differentiable functions,...
Tue May 10, 2011 3:10 am UTC
Forum: Computer Science
Topic: Ray Tracer Project
Replies: 18
Views: 3072

### Re: Ray Tracer Project

Thanks to you both, the Ray Tracer it is

Also if anyone has any nice articles on it they'd be helpful. I already have a few, but the more the better.
Thanks!
Thu May 05, 2011 6:42 pm UTC
Forum: Computer Science
Topic: Ray Tracer Project
Replies: 18
Views: 3072

### Ray Tracer Project

I'm in second year Computer Science and I have to choose a project. The prof gave the ray tracer as an example (or maybe ray caster, he just basically said "take a 3d picture") an it seems pretty cool. Do you think it's feasible to write this? I may be in the same group with two other stud...
Mon Mar 14, 2011 10:02 pm UTC
Forum: Mathematics
Topic: [0,1] ~ [0,1) (Cardinality)
Replies: 5
Views: 4148

### [0,1] ~ [0,1) (Cardinality)

I've been trying to prove that [0,1] is equipotent to [0,1) by trying to find a bijection between them and had no luck. I then asked my teacher and he told me that I should find an injection from each to the other, as that guarantees they are equipotent. Now I have two questions: Where can I find a ...
Mon Jun 07, 2010 8:29 pm UTC
Forum: Mathematics
Topic: (Proof of) Existence of Greatest Common Divisor
Replies: 15
Views: 5958

### Re: (Proof of) Existence of Greatest Common Divisor

Doraki wrote:Well if they have a common divisor d > 1, a1 = a2*d and b1 = b2*d, then a = a2*(d*g) and b = b2*(d*g),
so (d*g) is a common divisor of a and b and is greater than g.

Ooooh! Thanks!
Mon Jun 07, 2010 8:07 pm UTC
Forum: Mathematics
Topic: (Proof of) Existence of Greatest Common Divisor
Replies: 15
Views: 5958

### Re: (Proof of) Existence of Greatest Common Divisor

I suppose you could proceed by contradiction, arguing that if there were some x in S such that not x|G, then lcm(x,G) would be a common divisor of both a and b (and hence a member of S) that was strictly larger than G, contrary to G's choice as the maximum element of S. As a stylistic point, it is ...
Mon Jun 07, 2010 7:14 pm UTC
Forum: Mathematics
Topic: (Proof of) Existence of Greatest Common Divisor
Replies: 15
Views: 5958

### (Proof of) Existence of Greatest Common Divisor

(In case the notation isn't common or unfamiliar to someone, when I write x | y it means "x divides y", or that there exists an integer c such that y = cx.) In the algebra course I'm taking the prof gave us a proof that the Greatest common divisor of two integers always exists and is uniqu...
Fri May 28, 2010 3:09 am UTC
Forum: Mathematics
Topic: Math discovered or invented?
Replies: 110
Views: 18265

### Re: Math discovered or invented?

The best word I've found is 'revealed'.
Fri Apr 23, 2010 6:41 pm UTC
Forum: Mathematics
Topic: Integral by u Sub.
Replies: 30
Views: 3602

### Re: Integral by u Sub.

Doing it by u-sub is needlessly complicated, and that's where your error could have been. Take u = x+1 \int_0^1 (x+1) dx = \int_1^2 u du = \frac{2^2}{2} - \frac{1^2}{2} = 2 - \frac{1}{2} = \frac{3}{2} Which is the correct result, not 24. It would have been easier, and clearer, to just split ...
Wed Apr 21, 2010 8:33 pm UTC
Forum: Mathematics
Topic: Wanted: punch line for joke
Replies: 15
Views: 3578

### Re: Wanted: punch line for joke

Maybe you all thought of this and discarded it. But how about..

Spoiler:
All of them!
Tue Mar 30, 2010 6:19 pm UTC
Forum: Science
Topic: Silence Making Device
Replies: 8
Views: 890

### Re: Silence Making Device

uncivlengr wrote:Are you thinking of something other than noise cancelling headphones?

*embarassment*

I thought those just had really good isolation.
Sorry!
Tue Mar 30, 2010 6:10 pm UTC
Forum: Science
Topic: Silence Making Device
Replies: 8
Views: 890

### Re: Silence Making Device

It is possible to make a device that makes silence in one specific spot only, because your counter-waves will only cause destructive interference in one spot (unless you try to eliminate a plane wave, but that is rather rare with sound) But, say I have a tiny device that inputs a wave and outputs i...
Tue Mar 30, 2010 5:55 pm UTC
Forum: Science
Topic: Silence Making Device
Replies: 8
Views: 890

### Silence Making Device

Is it possible to create a device that makes silence?

The first thought that comes to mind is taking the sound in by microphone, reversing the wave and emitting it, which would create silence near the device. Now, does this work? Theoretically? Practically?

Any other ideas?

Thanks!
Mon Mar 01, 2010 1:03 am UTC
Forum: Mathematics
Topic: Man, fuck the "arbitrary constant"!
Replies: 14
Views: 2462

### Re: Man, fuck the "arbitrary constant"!

Wed Aug 05, 2009 12:22 pm UTC
Forum: Science
Topic: Making a lava lamp
Replies: 12
Views: 5505

### Re: Making a lava lamp

Much sounder plan. Even better than perc might be petroleum jelly, which is basically just really long chain hydrocarbons. You may need to heat the wax and petroleum jelly to combine them, but the same idea should apply. Petroleum jelly is just probably cheaper. Take pictures! Also, if you do your ...
Tue Aug 04, 2009 9:44 pm UTC
Forum: Science
Topic: Making a lava lamp
Replies: 12
Views: 5505

### Re: Making a lava lamp

Well, you succeded in scaring me with the alcohol/mineral oil lamp, so I decided to build a wax one. Basic formula is water and a mix of wax and perchloroethylene as the lava. The perchloroethylne (from now on, perc) is for making the wax denser, and getting it as close as possible to the density of...
Thu Jul 30, 2009 6:24 am UTC
Forum: Science
Topic: Making a lava lamp
Replies: 12
Views: 5505

### Re: Making a lava lamp

I don't know much about these chemicals, but isn't that a fire hazard? It's a GREAT fire hazard. That's why I will be extremely careful and test the thing thoroughly. I think the "real" lava lamps use wax that melts when it gets to the temperature of the light bulb you're using to heat it...
Wed Jul 29, 2009 11:40 pm UTC
Forum: Science
Topic: Making a lava lamp
Replies: 12
Views: 5505

### Making a lava lamp

How should go about it? My ingredients will be a mixture of 70% and 90% alcohol (which I think is the volume percentage of ethanol on water) and mineral oil. I'm planning to use the lamp alone to heat the lava and having a wire coil at the bottom to break the surface tension. Any suggestions, tips o...
Wed Jul 29, 2009 10:45 pm UTC
Forum: Science
Topic: Constant Acceleration Ship?
Replies: 10
Views: 3301

### Re: Constant Acceleration Ship?

If you've got a ship with a 1.5 TW constant power drive system, would the ship have a constant acceleration? First off, according to the people on the ship, and then according to a distant observer. Assume non-relativistic speeds, an unchanging ship mass, and in a vacuum. I keep getting that the pe...
Fri Apr 10, 2009 12:44 am UTC
Forum: Science
Topic: Global Warming and sea level rise
Replies: 34
Views: 3577

### Global Warming and sea level rise

I was thinking about that physics puzzle where you have a block of ice in a glass filled with water and need to calculate the rise in the water level. If the block of ice isn't touching the bottom and is being held at the top only by buoyancy then the level doesn't change at all. Then.. is the ice i...
Sun Mar 22, 2009 9:17 pm UTC
Forum: Science
Topic: Plastic bottle materials
Replies: 6
Views: 858

### Plastic bottle materials

I need to write about what materials can be used for carbonated beverages bottles and I've been looking at the different polymers and have one question: Is any material other than Polyethylene terephthalate used? It seems like a great polymer for the job, it's cheap and 100% recyclable while providi...
Mon Feb 23, 2009 2:53 pm UTC
Forum: News & Articles
Topic: 'Racist' cartoon causes stir
Replies: 681
Views: 47001

### Re: 'Racist' cartoon causes stir

Why is it not right to inform a company that you dislike something they've done? That is EXACTLY HOW BUSINESS WORKS. The more drastic the means your customers take to inform you of something, the more dire your fuck-up. You can inform the company, and the newspaper. You just can't be fucking angere...
Mon Feb 23, 2009 12:58 pm UTC
Forum: News & Articles
Topic: 'Racist' cartoon causes stir
Replies: 681
Views: 47001

### Re: 'Racist' cartoon causes stir

Something can't be or not be racist. It's racist by it's intention and by it's interpretation.
You'll never know the intention, interprete it as you see fit.

That's what I think.

Oh and about the coca-cola thing, just because someone did it it doesn't make it right.
Sun Feb 22, 2009 8:38 pm UTC
Forum: News & Articles
Topic: 'Racist' cartoon causes stir
Replies: 681
Views: 47001

### Re: 'Racist' cartoon causes stir

Ok this thread is a fucking wall of text so let me say something that I don't know if it has been posted before. If the cartoonist made a racist cartoon. So what? He can draw whatever he wants. It is up to the newspaper to fire him or not. It is up to you if you buy the paper or not. If coca-cola ch...
Sun Feb 22, 2009 8:08 pm UTC
Forum: Science
Topic: You have a barometer...
Replies: 30
Views: 4129

### Re: You have a barometer...

Hit the people asking in the head with the barometer and run, RUN.

Then when you see the news about your little riot they will obviuosly mention the height of the building in which the event took place.

Victory.
Sun Feb 22, 2009 7:59 pm UTC
Forum: Science
Topic: Why are electrical outlets grounded?
Replies: 35
Views: 54393

### Re: Why are electrical outlets grounded?

Because they misbehaved.

No, really. I see the advantage of not having a big ass-cable that needs to transport three times the current of the other fases. Also I think the generator doesn't have a ground cable, so any one will do. There is no actual zero.

But I dunno.
Fri Feb 20, 2009 12:47 pm UTC
Forum: Mathematics
Topic: Favorite math jokes
Replies: 1452
Views: 494682

### Re: Favorite math jokes

PM 2Ring wrote:Do they serve beer in Klein steins at the Möbius strip club?

Once I tried to get into the Klein club but I couldn't...
Tue Feb 17, 2009 3:47 pm UTC
Forum: Science
Topic: Ideas for a 10-15 minute talk on some aspect of Physics.
Replies: 15
Views: 2036

### Re: Ideas for a 10-15 minute talk on some aspect of Physics.

Geometric optics is pretty cool, but when you start analyzing lenses it gets boring fast. The most interesting part is refraction and the principle of least time. Ever saw a star shining in colors? It happens because the air between you and the star changes temperature, so the refracgtive index chan...
Sun Feb 15, 2009 12:26 am UTC
Forum: Logic Puzzles
Topic: Hilbert's Hotel
Replies: 102
Views: 10849

### Re: Hilbert's Hotel

can you fit arbitrarily large infinities into the hotel, like a mothership containing an infinite number of fleets? And an infinite number of motherships? And so on? No, this wouldn't work. In fact, it wouldn't even work if each bus only contained 2 people, and each spaceship only contained 2 buses...
Sat Feb 14, 2009 10:58 pm UTC
Forum: Logic Puzzles
Topic: Hilbert's Hotel
Replies: 102
Views: 10849

### Re: Hilbert's Hotel

I didn't read this whole thread, so sorry if this has already been discussed, but... What if a fleet of an infinite amount of starships arrives, each with an infinite amount of buses, each with an infinite amount of people? Can you house them all in the hotel? If so, can you fit arbitrarily large i...
Fri Feb 13, 2009 1:07 am UTC
Forum: General
Topic: I'm so nerdy that I ...
Replies: 1377
Views: 206123

### Re: I'm so nerdy that I ...

I'm so nerdy that I have a prime cell phone number. Well, it's prime if you don't count the country code (1). I wish I could say it was intentional, but it was entirely by accident. There's also the fact that I bothered to check in the first place. You just made me realize I have a prime cell phone...
Sun Feb 08, 2009 2:12 am UTC
Forum: General
Topic: Racism [Split]
Replies: 89
Views: 5993

### Re: Racism [Split]

I'm just gonna go ahead and post what's been on my mind for the past month or so. Racism is wrong because it is based in the belief that you can characterize all the members in a certain group from only part of it's integrants. This is also the base of other awful things, like sports fights, sexism,...
Sat Feb 07, 2009 10:02 pm UTC
Forum: Logic Puzzles
Topic: Cab fare
Replies: 4
Views: 1749

### Re: Cab fare

notgm wrote:why not just have the cabbie split the bill at each stop, and start anew as you drive away?

Don't you get a fixed fare at the start and then you get charged for time/distance? Works this way here.
Sat Feb 07, 2009 9:13 pm UTC
Forum: Mathematics
Topic: What is the smallest positive integer with no google hits?
Replies: 34
Views: 4922

### Re: What is the smallest positive integer with no google hits?

Here's an upper limit: 387462325
I just slammed my keyboard and nothing showed up.
Sat Feb 07, 2009 2:51 am UTC
Forum: Science
Topic: Physics Weight Question
Replies: 16
Views: 1480

### Re: Physics Weight Question

Thing is. To keep the weighs at rest, you not only have to cancel the external forces but also the external torques . So if the center of mass of the weighs isn't between your hands you'll need to apply more force (some from below, some from above, like when you hold a long stick by it's tip and you...
Fri Feb 06, 2009 11:46 pm UTC
Forum: Logic Puzzles
Topic: Hilbert's Hotel
Replies: 102
Views: 10849

### Re: Here's a fun problem.

Arrays start at 0. Heretic! Arrays should start at one. *Moving thread to the Holy Wars section* Good god... you don't actually believe that?? Anyway, not the point! Govalant: what about the 5th person in the 4th bus? He goes in room 14. Or 15 for nahkh :P. In your code, he goes in room infinity. :...
Fri Feb 06, 2009 11:30 pm UTC
Forum: Logic Puzzles
Topic: Hilbert's Hotel
Replies: 102
Views: 10849

### Re: Here's a fun problem.

Actaeus wrote:
nahkh wrote:
Govalant wrote:Arrays start at 0.

Heretic! Arrays should start at one.
*Moving thread to the Holy Wars section*

Good god... you don't actually believe that??

Anyway, not the point!

Govalant: what about the 5th person in the 4th bus?

He goes in room 14. Or 15 for nahkh .