## Search found 2051 matches

- Thu Oct 08, 2015 5:25 am UTC
- Forum: Logic Puzzles
- Topic: Timed Bridge Problem
- Replies:
**5** - Views:
**3106**

### Re: Timed Bridge Problem

If Adam is responsible for holding the lantern the entire time, you get AD A AC A AB, for 2A + B + C + D. This could be faster, or it could not--the specific times aren't given, so we have no way of knowing whether 2B is faster or slower than A+C. We would need to know this in order to determin...

- Mon Oct 05, 2015 1:48 pm UTC
- Forum: Mathematics
- Topic: Solving diophantine equation
- Replies:
**5** - Views:
**1079**

### Re: Solving diophantine equation

Hi, Here is a polynomial P(k) P(k)=2*(a+k)!-k^2-k(2b+1)-b+6 where a and b are positive integers >0 Assuming that P(k) could be negative for some (k0,a,b) and positive for some (k1,a,b) can we state or claim that P(k)=0 has at least one solution. Thank you for your thoughts. No. Firstly, P(k) is not...

- Fri Oct 02, 2015 10:39 pm UTC
- Forum: Logic Puzzles
- Topic: Caesar cipher
- Replies:
**3** - Views:
**1908**

### Re: Caesar cipher

**Spoiler:**

- Fri Oct 02, 2015 12:30 pm UTC
- Forum: Logic Puzzles
- Topic: How many rooms are there in the tower?
- Replies:
**26** - Views:
**4584**

### Re: How many rooms are there in the tower?

This reminds me of a technique I used to use occasionally in a painting program. If there was an area that had been dithered (i.e. was coloured in some pattern using two distinct colours) and I wanted it to be just one colour, I would alternate floodfilling it with one colour and then the o...

- Wed Sep 30, 2015 9:17 am UTC
- Forum: Science
- Topic: Speed of waves in water
- Replies:
**8** - Views:
**3370**

### Re: Speed of waves in water

Googling gave me this site that has an idealised formula: http://hyperphysics.phy-astr.gsu.edu/hbase/watwav.html#c3 This says that for deep water, wave speed becomes proportional to sqrt(wavelength), but for shallow water the speed becomes independent of wavelength. So, water waves do travel at dif...

- Tue Sep 29, 2015 11:26 am UTC
- Forum: Science
- Topic: Speed of waves in water
- Replies:
**8** - Views:
**3370**

### Re: Speed of waves in water

Googling gave me this site that has an idealised formula:

http://hyperphysics.phy-astr.gsu.edu/hb ... av.html#c3

This says that for deep water, wave speed becomes proportional to sqrt(wavelength), but for shallow water the speed becomes independent of wavelength.

http://hyperphysics.phy-astr.gsu.edu/hb ... av.html#c3

This says that for deep water, wave speed becomes proportional to sqrt(wavelength), but for shallow water the speed becomes independent of wavelength.

- Mon Sep 28, 2015 7:00 am UTC
- Forum: Mathematics
- Topic: Theoretical problem about traveler salesman
- Replies:
**6** - Views:
**1764**

### Re: Theoretical problem about traveler salesman

The problem doesn't get much easier when you know the exact length of the shortest path ahead of time. You can prove this by imagining you had a magic box (aka oracle) which could solve the problem if it was told ahead of time the exact length of the shortest path, and then use the magic box to sol...

- Wed Sep 23, 2015 5:24 pm UTC
- Forum: Mathematics
- Topic: geometry [looking at triangles in perspective]
- Replies:
**7** - Views:
**1706**

### Re: geometry [looking at triangles in perspective]

It does seem to be a bit tricky. but you've got a sound argument. Actually, I don't. For obtuse triangles my argument does not work, because my #2 should have read "the intersection of any two of the spheres must not lie wholly within the third". For this case you do need to use a smaller...

- Wed Sep 23, 2015 2:54 pm UTC
- Forum: Mathematics
- Topic: geometry [looking at triangles in perspective]
- Replies:
**7** - Views:
**1706**

### Re: geometry [looking at triangles in perspective]

It does seem to be a bit tricky. I think you can simplify things by choosing 90 degrees as your viewing angle, i.e. try to find point P such that angles APB, BPC, CPA are all 90 degrees. The points in 3d space such that angle APB is 90 degrees is exactly the sphere that has AB as its diameter. The s...

- Fri Sep 11, 2015 10:01 am UTC
- Forum: Mathematics
- Topic: I don't seem to understand my own brain teaser, please help!
- Replies:
**22** - Views:
**4199**

### Re: I don't seem to understand my own brain teaser, please h

The expected value is positive because you can never lose more than 10 days but if you win you can win high. Exactly. Even just playing twice, you have: 0.55^2 = 30.25% probability of getting 4 times your initial amount 2*0.55*0.45 = 49.5% probability of getting 0.6 times your initial amount 0.45^2...

- Wed Sep 09, 2015 8:52 am UTC
- Forum: Logic Puzzles
- Topic: Fill square puzzle
- Replies:
**4** - Views:
**1734**

### Re: Fill square puzzle

The intended answer is of course:

**Spoiler:**

- Thu Aug 27, 2015 3:29 pm UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**9743** - Views:
**1555112**

### Re: Coding: Fleeting Thoughts

Open that in Chrome, or IE, you get a text box with some words after it. But open it in Firefox, and you just get an ellipsis. Except the text box is still there - mouse over where it should be and you get the text-edit cursor, and click and it'll focus the text box, which will suddenly appear agai...

- Fri Aug 21, 2015 6:11 am UTC
- Forum: Language/Linguistics
- Topic: ei <-> ie: English speciality?
- Replies:
**21** - Views:
**4774**

### Re: ei <-> ie: English speciality?

Derek wrote:gmalivuk wrote:For me at least, weird definitely rhymes with tiered.

I agree, and stand by my original question?

Weird rhymes with tiered/feared, but this is not the same sound as feel/receive/protein.

Just like peer and peel have different sounds.

(For me at least - I have a BrE accent.)

- Thu Aug 20, 2015 9:16 am UTC
- Forum: Logic Puzzles
- Topic: Three Cards Trick
- Replies:
**15** - Views:
**2318**

### Re: Three Cards Trick

I have read of a legitimate variant of this trick, but that involved 5 cards: - 5 cards are freely chosen from a deck of 52. - The Magician's assistant chooses one of these 5 cards, and lays the other 4 face up in a row. - Those 4 cards are read out by a spectator to the magician on the phone. - Mag...

- Wed Aug 19, 2015 2:27 pm UTC
- Forum: Language/Linguistics
- Topic: ei <-> ie: English speciality?
- Replies:
**21** - Views:
**4774**

### Re: ei <-> ie: English speciality?

Well, English does have words with ei and words with ie where those vowels are pronounced the same, e.g. receive and reprieve. For that vowel sound there is the general rule that after the letter c it's ei and otherwise it's ie, but there are some exceptions. I think that many English people are tau...

- Fri Aug 07, 2015 8:43 am UTC
- Forum: Science
- Topic: A million years
- Replies:
**59** - Views:
**7246**

### Re: A million years

Well, kelvin is a unit of temperature, not intensity. Intensity is the power per area per solid angle . . . per wavelength (W·sr -1 ·m -3 in SI, though you won't usually see it written that way; unfortunately intensity has many definitions in many contexts). In this sense, it is not immediately rel...

- Wed Aug 05, 2015 8:06 pm UTC
- Forum: Mathematics
- Topic: Conjecture about odd prime numbers
- Replies:
**5** - Views:
**1597**

### Re: Conjecture about odd prime numbers

After thinking about it a bit more, you can always take a=(p-1)/2 and b=0.

From Fermat's little theorem, 2^(p-1) == 1 modulo p. Therefore 2^a with a=(p-1)/2 is a number that when squared results in 1 modulo p, so 2^a is 1 or -1.

From Fermat's little theorem, 2^(p-1) == 1 modulo p. Therefore 2^a with a=(p-1)/2 is a number that when squared results in 1 modulo p, so 2^a is 1 or -1.

- Wed Aug 05, 2015 3:56 pm UTC
- Forum: Mathematics
- Topic: Conjecture about odd prime numbers
- Replies:
**5** - Views:
**1597**

### Re: Conjecture about odd prime numbers

Every odd prime number p can be expressed at least once as : p=(2^a + or - 2^b)/k where a and b > 0 a+b < p k some integer > 0 Examples : 5=2^2+2^0 7=2^3-2^0 11=(2^5+2^0)/3 23=(2^11-2^0)/89 Look at the values of +-2^n modulo p. For example with p=11 we have 2^0 == 1, -2^0 == 10 2^1 == 2, -2^1 == 9 ...

- Sat Aug 01, 2015 6:52 am UTC
- Forum: Mathematics
- Topic: I don't seem to understand my own brain teaser, please help!
- Replies:
**22** - Views:
**4199**

### Re: I don't seem to understand my own brain teaser, please h

It is not really valid to take the (weighted arithmetic) average of two multiplication factors to get the expected multiplication factor. A simpler example would be a 50:50 chance of doubling or halving, which should be neutral in the long run, and not have an average muliplication factor of (2+.5)/...

- Thu Jul 30, 2015 8:29 pm UTC
- Forum: Mathematics
- Topic: New type of pentagonal tile
- Replies:
**6** - Views:
**29871**

### Re: New type of pentagonal tile

My thoughts, in unspecified order: 1. interesting 2. not all text is readable due to the resolution 3. the tiling pattern is pretty complex 4. this isn't published yet, is it? 5. the tile in the picture has a specific combination of values, no independent variable (except for scale obviously). Is i...

- Thu Jul 30, 2015 12:50 pm UTC
- Forum: Mathematics
- Topic: New type of pentagonal tile
- Replies:
**6** - Views:
**29871**

### New type of pentagonal tile

A new type of tileable pentagon has been found! There are many kinds of tiling that use just one shape of pentagonal tile (and its mirror image if necessary). Up to now, these tiles fell into 14 types (see http://www.mathpuzzle.com/tilepent.html ). Now another has been found. type15.png I did a lot ...

- Fri Jul 24, 2015 2:45 pm UTC
- Forum: Mathematics
- Topic: Math: Fleeting Thoughts
- Replies:
**378** - Views:
**101370**

### Re: Math: Fleeting Thoughts

I've been reading graph theory -- the definition of adjacency keeps giving me trouble. Everything I read describes adjacency as one of two things: Vertices connected by an edge, or vertices that share an edge. However, both 'connected' and 'shared' confuse me when it comes to directed edges: If X a...

- Sat Jul 18, 2015 9:53 pm UTC
- Forum: Mathematics
- Topic: sum and product of natural number
- Replies:
**9** - Views:
**2405**

### Re: sum and product of natural number

snip If B says yes to the first question, doesn't that mean it could be any number with only two divisors? If B says no then that excludes a=b=1; all a=p, b=1; a=p1, b=p2. Where p, p1, p2 are prime and p1=!p2 The number p1*p2 has four divisors, namely 1, p1, p2, and p1*p2 - there are two ways t...

- Fri Jul 17, 2015 8:38 am UTC
- Forum: Mathematics
- Topic: Pairwise Differences
- Replies:
**3** - Views:
**1385**

### Re: Pairwise Differences

The set of numbers evenly divisible by n is closed under talking the set of pairwise differences. Are there any sets of natural numbers with this property that are not subsets of any set in the above class? In particular, I'm interested in those that, like the above, can be truncated to n elements ...

- Thu Jul 16, 2015 5:05 am UTC
- Forum: Language/Linguistics
- Topic: Words You Didn't Know That Describe Things Perfectly
- Replies:
**91** - Views:
**23492**

### Re: Words You Didn't Know That Describe Things Perfectly

Cornobble (archaic): to hit with a fish. Can anyone provide any reference for this being a real thing? The OED online says it isn't, and I can't find any reputable source. I heard it mentioned on the "Says You" podcast, and the host claims to have read it in a book, but I am dubious. I ha...

- Wed Jul 15, 2015 12:03 pm UTC
- Forum: Coding
- Topic: Coding: Fleeting Thoughts
- Replies:
**9743** - Views:
**1555112**

### Re: Coding: Fleeting Thoughts

I really hate that C++ calls its dynamic array a "vector" - then when I want to use my own type, I have to call it a column_matrix to avoid confusion, even though my type - an immutable, fixed-length, passed by value array that supports scalar addition and xor, scalar multiplication, and ...

- Tue Jul 14, 2015 10:51 am UTC
- Forum: Science
- Topic: Conductors and Motors/Generators
- Replies:
**7** - Views:
**1621**

### Re: Conductors and Motors/Generators

Rambling on, it occurred to me that a liquid would be awkward in terms of the brushes, so why not make the winding rigid and allow the magnets to rotate. Then I wondered why all the motors I've seen always rotate the winding - wouldn't it be easier in general to rotate the magnets and avoid the nee...

- Thu Jul 09, 2015 4:15 pm UTC
- Forum: Mathematics
- Topic: Marks and Integers
- Replies:
**3** - Views:
**1403**

- Wed Jul 08, 2015 7:20 am UTC
- Forum: Mathematics
- Topic: Wanted: Elegant proof of sin(x)/x limit
- Replies:
**49** - Views:
**6962**

### Re: Wanted: Elegant proof of sin(x)/x limit

Qaanol wrote:jestingrabbit wrote:I claim |BD| < |BE|+|ED|

<snip>

verified by good sense

That’s the part I’m struggling to prove cleanly.

This is the Triangle Inequality and that wiki page has a short euclidean proof.

- Sun Jul 05, 2015 3:27 pm UTC
- Forum: Mathematics
- Topic: Factorial expressed as sum of consecutive numbers
- Replies:
**25** - Views:
**3607**

### Re: Factorial expressed as sum of consecutive numbers

Thank you for your comments and computations. Now we assume that the maximal length of the consecutive positive numbers is solved. So we could express n!=S(f,l) where f is the first number of the sequence and l the last number of the sequence. S(f,l) is the summation. The next step is computing S(f...

- Fri Jul 03, 2015 9:40 pm UTC
- Forum: Mathematics
- Topic: Factorial expressed as sum of consecutive numbers
- Replies:
**25** - Views:
**3607**

### Re: Factorial expressed as sum of consecutive numbers

Can we conjecture that as n grows the length is ALWAYS an odd number? A factorial can be expressed as equal to A*B where A = 2^k and B an odd number. Limit of the ratio A/B = 0 as n grows to infinity. Does it imply that the length will be always odd? No. You asked for the longest such sequence, whi...

- Fri Jul 03, 2015 1:05 pm UTC
- Forum: Mathematics
- Topic: Factorial expressed as sum of consecutive numbers
- Replies:
**25** - Views:
**3607**

### Re: Factorial expressed as sum of consecutive numbers

There is still one hurdle to get over : the complexity. As n grows can we apply this algorithm with minimal complexity. What will be the complexity if n is 500 digits number? I`m not programmer so I just want to know if we can do it in reasonable time. If n is 500 digits, i.e. is about 10^500, then...

- Fri Jul 03, 2015 12:18 pm UTC
- Forum: Mathematics
- Topic: Factorial expressed as sum of consecutive numbers
- Replies:
**25** - Views:
**3607**

### Re: Factorial expressed as sum of consecutive numbers

Here's a way to do it. Take 2*n!, and write it as a product a*b, with: - a and b of differing parities (i.e. one is odd and one is even but it does not matter which is which) - a and b as close to each other as possible, (i.e. as near to sqrt(2*n!) as possible) - a<b Then n! is equal to the sum of a...

- Thu Jun 18, 2015 5:53 am UTC
- Forum: Coding
- Topic: Happy bases
- Replies:
**12** - Views:
**3045**

### Re: Happy bases

Find a more efficient algorithm to determine whether a number is happy in a given base. One thing I've noticed is that you immediately forget all previous calculations. If you've verified all numbers from 1 to n-1 to be happy, you only need to calculate the sequence for n until reaching any number ...

- Tue Jun 09, 2015 12:16 pm UTC
- Forum: Mathematics
- Topic: Independent repeated trials and mutually exclusive events.
- Replies:
**4** - Views:
**1637**

### Re: Independent repeated trials and mutually exclusive event

Mutually exclusive is just a fancy term for saying that there is no overlap between the events. For instance, if I wanted to know the number of people in my class who are either women or math majors, I can't add the number of women to the number of math majors because that would double-count all th...

- Sat Jun 06, 2015 4:24 pm UTC
- Forum: Coding
- Topic: Project Euler
- Replies:
**411** - Views:
**141618**

### Re: Project Euler

@jaap: Would you mind referring me to a simple memoized implementation of a factorial method in Java? That would probably help me a lot, just seeing how the memoization is implemented.... Actually, maybe I do know enough to implement it. I suppose you would have to have it in its own class so a fie...

- Wed Jun 03, 2015 2:53 pm UTC
- Forum: Coding
- Topic: Reflection around the inside of a circle
- Replies:
**4** - Views:
**2787**

### Re: Reflection around the inside of a circle

I always found it easier to do this kind of thing with vectors, I always make mistakes with trigonometric calculations. And calculating a normal vector for a sphere is really easy. I totally agree. In most languages I've seen, atan2 has the y parameter first, so reflectTangent isn't actually the ta...

- Tue Jun 02, 2015 10:41 pm UTC
- Forum: Mathematics
- Topic: Combinations algorithm and property
- Replies:
**19** - Views:
**2588**

### Re: Combinations algorithm and property

The loop part (that the last one is the neighbour of the first one) makes this tricky. Without that part I would solve it by methodically moving freespaces (assuming I didn't misunderstand something). That does indeed work as far as I can tell for a non-circular list. Here is some Java code that do...

- Tue Jun 02, 2015 3:19 pm UTC
- Forum: Mathematics
- Topic: Combinations algorithm and property
- Replies:
**19** - Views:
**2588**

### Re: Combinations algorithm and property

In other words - A set with n elements has nCk subsets of k elements. Find a way to put these subsets in a (circular) list such that each adjacent pair of subsets in the list have k-1 elements in common. Preferably find an algorithm that allows you to generate such a list. This feels a bit like a Gr...

- Tue Jun 02, 2015 11:52 am UTC
- Forum: Coding
- Topic: Project Euler
- Replies:
**411** - Views:
**141618**

### Re: Project Euler

This can overflow in that multiplication in cases where your version would not, but if you are worried about that you can use the gcf function to cast out common factors between i and c, and between i and (n+1-i) before multiplying, just like in your Fraction.times function. This is a great point. ...