## Search found 825 matches

- Wed Mar 16, 2011 2:17 am UTC
- Forum: Mathematics
- Topic: 0÷0
- Replies:
**22** - Views:
**2748**

### Re: 0÷0

Arguing on the internet, how did I fall into this trap again?

- Wed Mar 16, 2011 1:51 am UTC
- Forum: Mathematics
- Topic: 0÷0
- Replies:
**22** - Views:
**2748**

### Re: 0÷0

The square root thing you demonstrate doesn't require us to modify any fundamental axioms, nor does it break most of higher mathematics. It doesn't require us to drop any of the field axioms, but a priori, there's no reason they should be our only axioms. One could imagine a different set of axioms...

- Tue Mar 15, 2011 4:07 pm UTC
- Forum: Mathematics
- Topic: Matrix multiplication seems so arbitrary
- Replies:
**19** - Views:
**3842**

### Re: Matrix multiplication seems so arbitrary

Linear algebra is really about linear transformations. By the properties of linearity, a linear transformation is uniquely defined by what it does to the unit vectors, because if you know that f((1,0))=u and f((0,1))=v, then you know that f((a,b))=f((a,0))+f((0,b))=af((1,0))+bf((0,1))=au+bv (I reall...

- Tue Mar 15, 2011 2:02 pm UTC
- Forum: Mathematics
- Topic: 0÷0
- Replies:
**22** - Views:
**2748**

### Re: 0÷0

I disagree with your first idea, that what I did wasn't really division. The thing is that it's really similar to calling i the square root of -1, No. It is not similar at all. The existence of a square root of -1 does not violate any of the laws of algebra. The existence of 1/0 would do so. Yes it...

- Mon Mar 14, 2011 11:08 pm UTC
- Forum: Mathematics
- Topic: [0,1] ~ [0,1) (Cardinality)
- Replies:
**5** - Views:
**3473**

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

It's easy to find an explicit bijection. All you want to do is to remove a single point from your set and show that your new set has the same cardinality. This should be familiar to you: how do you show that {0,1,2,3,...} has the same size cardinality as {1,2,3,...}? A bijection in this case is f(n)...

- Sun Sep 26, 2010 8:11 am UTC
- Forum: Mathematics
- Topic: Probablity
- Replies:
**3** - Views:
**642**

- Sat Aug 14, 2010 10:03 am UTC
- Forum: Mathematics
- Topic: Does anyone else create formulae on the spot?
- Replies:
**21** - Views:
**2336**

### Re: Does anyone else create formulae on the spot?

I don't. I consider linear formulopodes too trivial and non-linear ones too difficult.

- Tue Aug 10, 2010 7:19 pm UTC
- Forum: Mathematics
- Topic: question about metrical spaces
- Replies:
**3** - Views:
**733**

- Tue Aug 10, 2010 7:16 pm UTC
- Forum: Mathematics
- Topic: Lectures for my mp3 player
- Replies:
**3** - Views:
**747**

### Re: Lectures for my mp3 player

I don't think that's possible or that such a thing exists. You can't listen to a lecture the same way you'd listen to an audiobook. You need time to reflect on the ideas, and also it's much easier if you can read the notation on a blackboard. What I suggest instead is that you find a mathematical pr...

- Mon Aug 09, 2010 5:09 pm UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

Alright, that (almost) works. I'd be interested in if an actual tiling can be made, but your construction works if we massage the definitions a little: In my opinion, a tiling should be a partition of a set. So no overlapping of edges. Thus, we should ask if we can tile the "half-open" imp...

- Mon Aug 09, 2010 3:30 pm UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

Qaanol wrote:Spoiler:

Why should this be true? You can't tile a square with disks (no disk, no matter how small, can ever cover a corner), so why should these rectangles be able to do the job?

- Mon Aug 09, 2010 2:07 am UTC
- Forum: Logic Puzzles
- Topic: A new kind of hat puzzle!
- Replies:
**2** - Views:
**1739**

### A new kind of hat puzzle!

I'm loving the recent surge in puzzle threads here! I'll contribute one of my most recent favorite puzzles. I'm posting it in this forum because I think it requires more than just a trick to solve. During the years I've seen lots of hat puzzles on these forums, but I haven't seen this one. I saw thi...

- Mon Aug 09, 2010 1:13 am UTC
- Forum: Logic Puzzles
- Topic: Colliding balls
- Replies:
**7** - Views:
**1601**

### Re: Colliding balls

Oh, you're right. Yes, that was what I was thinking of, but I realize now that's not what I wrote in the problem statement.

- Mon Aug 09, 2010 1:09 am UTC
- Forum: Logic Puzzles
- Topic: Colliding balls
- Replies:
**7** - Views:
**1601**

### Re: Colliding balls

I mean the left one starts out by going right and the right one starts out by going left, so they will collide with eachother and both go off the table. You need to contract the interval in (2) just as you did in (3). Yay for edits: Well, take the radius as r/2 minus epsilon, and you...

- Mon Aug 09, 2010 1:02 am UTC
- Forum: Logic Puzzles
- Topic: Colliding balls
- Replies:
**7** - Views:
**1601**

### Re: Colliding balls

Sure! I solved them the same way. I thought the trick needed for (3) was neat. About your (2): Actually this will depend on the radius of the balls. Suppose you only have two balls, one on each edge of the table, precisely so large that they touch. They will both fall off instantaneously (not ta...

- Sun Aug 08, 2010 11:07 pm UTC
- Forum: Logic Puzzles
- Topic: Colliding balls
- Replies:
**7** - Views:
**1601**

### Colliding balls

Suppose you have n identical balls on a 1 meter long (one-dimensional) table, each with radius r and speed 1 m/s, but with different orientations (some go left, some go right). The table is frictionless and collisions are elastic. 1) At most how many collisions will there be before they all fall off...

- Sun Aug 08, 2010 10:40 pm UTC
- Forum: Mathematics
- Topic: Pairing off points: More surprising trickiness
- Replies:
**18** - Views:
**1971**

### Re: Pairing off points: More surprising trickiness

antonfire & Talith: Very elegant variant of OverBored's solution.

- Sun Aug 08, 2010 10:36 pm UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

Torn Apart By Dingos: There checkerboard argument works to rule out an infinite tiling. The path argument carries through as well, with a bit of thought. Sure, there are infinite number of vertices, but the ones you can reach can only have whole-number displacements, of which there can be a finite ...

- Sun Aug 08, 2010 12:12 am UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

aleph_one: Nice trick! Talith: Thank you for making to reexamine my counterexample, I had given up on that approach. :) (Though I thought that you could do it by only going right and up from the bottom left corner, which I had a counterexample to - but when I examined it, I realized that it might wo...

- Sat Aug 07, 2010 10:56 am UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

OverBored: A 3x3 rectangle can be decomposed to nine 1x1 rectangles, and each of these have equal amounts of white and black no matter how we offset them. I think that proof is fine, and the upside of that one is that it doesn't need to assume a finite tiling (it assumes there are no rotated rectang...

- Fri Aug 06, 2010 6:47 pm UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

Talith: I had that idea too, but it doesn't seem to be true. NEW EDIT: That approach seems to work! Proof: Assume that a big rectangle is tiled by finitely many proper rectangles. I'll prove that we can move along a path from one corner to another along full integer sides of proper rectangle...

- Fri Aug 06, 2010 1:22 pm UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

What do you mean by "done the same way"? Yes, a hypothetical tiling using rotated rectangles.

- Fri Aug 06, 2010 1:00 pm UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

I don't understand your reasoning.Syrin wrote:No, because an infinite case is identical to a finite case (as it must be tiled by proper rectangles, and at least one side of a proper rectangle is at least 1)

- Fri Aug 06, 2010 11:44 am UTC
- Forum: Mathematics
- Topic: Tiling a rectangle: Another surprisingly tricky problem
- Replies:
**45** - Views:
**5028**

### Re: Tiling a rectangle: Another surprisingly tricky problem

Do we need to worry about infinite tilings and/or rotated rectangles?

- Thu Jul 29, 2010 4:05 am UTC
- Forum: Mathematics
- Topic: Polygon inside a polygon: A surprisingly tricky problem
- Replies:
**20** - Views:
**3087**

### Re: Polygon inside a polygon: A surprisingly tricky problem

I really like this puzzle and I look forward to seeing the second solution from you, aleph_one. Post more puzzles!

- Thu Jul 29, 2010 4:03 am UTC
- Forum: Logic Puzzles
- Topic: Cat and mouse
- Replies:
**29** - Views:
**12279**

### Re: Cat and mouse

I read another wording of this puzzle on a thread on mathoverflow and got exactly the same solution as aleph_one. The wording of the puzzle in that thread seems to imply that 2(N-2) is the best we can do. How could we prove this?

- Sun Jul 04, 2010 10:59 am UTC
- Forum: Mathematics
- Topic: Is it just me, or does the average guy really suck at math?
- Replies:
**57** - Views:
**6990**

### Re: Is it just me, or does the average guy really suck at ma

I was amused when one of the characters pondered for a moment before deciding 645 was not a prime.

- Wed Jun 09, 2010 9:29 am UTC
- Forum: Mathematics
- Topic: Probability Question
- Replies:
**14** - Views:
**1783**

### Re: Probability Question

Fix m=4, the length of the shorter string. Let x(n) be the number of strings of length n containing the shorter string as a substring (for now, we don't allow it be re-ordered). The first occurrence of the shorter string in a string of length n is after at some position i such that 0<=i<=n-m (so tha...

- Mon Jun 07, 2010 4:12 pm UTC
- Forum: Mathematics
- Topic: Probability Question
- Replies:
**14** - Views:
**1783**

### Re: Probability Question

Probably pretty small anyway. This can be calculated, but you are asking the wrong question, for many reasons. 1. The probability depends on the given string. "11" will appear less often than "12" in a longer string. 2. The four digits don't appear in her phone number, or social ...

- Wed Jun 02, 2010 10:22 am UTC
- Forum: Mathematics
- Topic: Math discovered or invented?
- Replies:
**110** - Views:
**14586**

### Re: Math discovered or invented?

Is it safe to say the axioms are invented and the theorems and proofs are discovered? I dont think natural numbers exist outside of human experience (or the experiences other highly intelligent beings that can define numbers). Do other forms of life experience the natural numbers? What about jelly ...

- Mon May 10, 2010 11:56 am UTC
- Forum: Mathematics
- Topic: Taylor Series
- Replies:
**15** - Views:
**2386**

### Re: Taylor Series

This is an important point that has to be stressed. People often complain about Wikipedia being awful for learning math, but that's not the point of Wikipedia. Wikipedia, as its name suggests, is meant to be an encyclopedia. It's a reference. You wouldn't try to learn French from Wikipedia (even th...

- Mon May 10, 2010 5:27 am UTC
- Forum: Mathematics
- Topic: Taylor Series
- Replies:
**15** - Views:
**2386**

### Re: Taylor Series

Taylor series are essentially polynomials of infinite degree. Polynomials are the simplest functions to deal with, and therefore it is convienient to write functions as power series if possible. For one thing, it allows us to calculate numerically functions to arbitrary precision: it is unclear, for...

- Sun May 09, 2010 6:00 pm UTC
- Forum: Mathematics
- Topic: Quaternions and graphics programming
- Replies:
**11** - Views:
**1436**

### Re: Quaternions and graphics programming

Sorry, I was wrong, your vectors were already normalized.

- Sun May 09, 2010 1:57 pm UTC
- Forum: Mathematics
- Topic: Quaternions and graphics programming
- Replies:
**11** - Views:
**1436**

### Re: Quaternions and graphics programming

Thanks, I had read most of the wiki stuff but hadn't really been able to gather much off it. This makes much more sense now. So the euler rotations of 90 degrees about each axis would be represented with the quaternions (cos(\pi/2) + sin(\pi/2)(1, 0, 0), cos(\pi/2...

- Fri May 07, 2010 12:21 pm UTC
- Forum: Mathematics
- Topic: Can we truly prove anything?
- Replies:
**86** - Views:
**9466**

### Re: Can we truly prove anything?

wouldn't that make any proof we've ever made completely useless? Not really. All that means is that ZFC is a bad foundation and we'd find another one. This got me thinking. Would it be possible that all sufficiently powerful systems (in which we can do that math we want to do; Euclidean geometry do...

- Fri Apr 02, 2010 10:14 pm UTC
- Forum: Mathematics
- Topic: A Graph of gravity + friction motion
- Replies:
**7** - Views:
**1210**

### Re: A Graph of gravity + friction motion

Yes, this is a geometric series (look this up on wikipedia, there's a very simple derivation for the formula). If d=LINEAR_DAMPING, then [math]d^0+...+d^n=\dfrac{1-d^{n+1}}{1-d}.[/math]

- Fri Mar 26, 2010 7:15 pm UTC
- Forum: Mathematics
- Topic: amsthm
- Replies:
**2** - Views:
**782**

### Re: amsthm

Is this what you're looking for? I'm using the following. Apart from those below, you can use the built-in \begin{proof}...\end{proof}. \usepackage{amssymb, amsmath, amsfonts, amsthm} \theoremstyle{plain} \newtheorem{Theorem}[equation]{Theorem} \newtheorem{Lemma}[equation]{Lemma} \newtheorem{Proposi...

- Fri Mar 12, 2010 5:22 pm UTC
- Forum: Mathematics
- Topic: Four color theorem
- Replies:
**82** - Views:
**17522**

### Re: Four color theorem

The surrounding ones don't have to be different colors. They can be colored A,B,A,C.

The innermost five parts of the following diagram is the same as your example, and it's colored with four colors.

The innermost five parts of the following diagram is the same as your example, and it's colored with four colors.

- Sat Mar 06, 2010 9:27 am UTC
- Forum: Mathematics
- Topic: Chain email
- Replies:
**28** - Views:
**2783**

### Re: Chain email

Actually that was what I was referring to. Writing "a=b=2a=2b" (equality sign between equations) or "a=>2a/2" (arrow where there should be an equality sign). I've seen worse, but admittedly it's rare. You're right that it doesn't mean they don't understand equality, but I'd like ...

- Fri Mar 05, 2010 11:41 pm UTC
- Forum: Mathematics
- Topic: Chain email
- Replies:
**28** - Views:
**2783**

### Re: Chain email

This is not an equivalence relation, and it's not assignment. Writing it this way doesn't help the reader, it is just confusing and perpetuates a bad understanding of the equality sign. Seriously? Do you think anyone is confused about what equality is because of an unusual use in a puzzle? Do you r...