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#51 Re: This is Cool » Prime Numbers!!! » 2009-06-14 09:27:44

RickIsAnIdiot wrote:

Can you type Factorization(1111111111111111111); into Magma ? And it shows all the digit,s ?

If I type Factorization(1111111111111111111); into Magma the output is [ <1111111111111111111, 1> ]. You'll have to tell me if this is showing all the digits.

#52 Re: This is Cool » 7*11*13!!! » 2009-06-09 10:17:03

A result related to this discussion is the following:


Suppose a and n are integers with
and
.
If
is prime then a is even and n is a power of 2.

Proof
Certainly a should be even, since if a is odd then

is even and not 2.

Suppose n = st where s is odd and s>1. Then


Now
so
and so
is not prime.
Hence if
is prime then n cannot have any odd divisor other than 1 so n is a power of 2.


It follows from this result that

is divisible by
so 1001 is divisible by 11.

Of course, ganesh's example shows that the converse of this result is not true since

is not prime even though 10 is even and 4 is a power of 2.

#53 Re: This is Cool » Prime Numbers!!! » 2009-06-09 09:39:44

I think bobbym's point is that, even for computers, the calculation of large factorials is not a very good way of testing if numbers are prime.

At university, if I type Factorization(1111111111111111111); into Magma I can't even tell that there is a delay before it prints [ <1111111111111111111, 1> ] confirming that 1111111111111111111 is indeed a prime.

If I type Factorial(1111111111111111111); instead it refuses to do the calculation because 1111111111111111111 is too large.

#54 Re: Help Me ! » Number of Factors. » 2009-06-07 23:43:44

shocamefromebay wrote:

If

,
is not always odd.

For example, let x = 4.





which is odd.
It is not too hard to prove that any non-zero square has an odd number of factors.

shocamefromebay wrote:


If b is an odd number and both a and b are integers greater than ten but less than 99, find an expression for x using the variables a and b to make the statements above true.

Suppose I write


This is an expression for x using the variables a and b.



So this expression for x makes the statements true.

I really can't see how I have failed to find an expression for x using the variables a and b to make the given statements true.
The only problem I can see with this solution is that x is not an integer when a is even, but no solution is possible in that case.

shocamefromebay wrote:

I think that there may need to be many separate solutions to find the all the possible solutions for this problem

As I admitted in my first reply, the solutions I have given are not the only solutions, so I completely agree with you that there is more work to be done in order to find all possible solutions. However, until we can agree that I have provided some solutions to this problem I have no interest in looking for others.

#55 Re: Help Me ! » Number of Factors. » 2009-06-07 16:46:47


I'm not questioning you ability to read or copy, but suggesting that whoever set the problem may have made a mistake.
If
then it turns out that
is odd, so there can be no solution when a is even.

The problem is to find an expression for x in terms of a and b such that a certain relationship between these three quantities holds. The value of x is not uniquely determined by a and b, and in my solution I only try to find some value of x that works. It is always possible to choose x so that x+1 is a prime power, and I choose to do so.

As an example, suppose a=3 and b=1. For any prime p,

and
. Hence we can choose x to be p-1 for and prime p.

#56 Re: Help Me ! » Number of Factors. » 2009-06-07 12:11:56

I think the requirement in the second part should be that a is odd rather than b.

You have probably already shown in the first part that

In general

will depend on the prime factorisation of
.
If we assume that
is a prime power
then

so we should choose
and
can be any prime.
These are not the only solutions, but I do think they are the simplest.

#57 Re: Help Me ! » probability problem » 2009-05-28 14:26:05

If the first sentence means "There are 100 gadgets. 10 are defective." then the answer is

#58 Re: Help Me ! » nth root of n is irrational proof » 2009-05-27 10:55:31

and p and q are coprime so we must have

so n is not the nth power of an integer.

#59 Re: Help Me ! » Proof by Induction? » 2009-05-20 12:58:24

bobbym wrote:

I'm not following you. When x=3, f(3) = 121 a square.

This exactly illustrates Kurre's point. TheDude's post presents a proof that the polynomial

is not the square of a polynomial.
Nevertheless,
is a square.

#60 Re: Help Me ! » More Questions... » 2009-05-19 09:55:51

1. The argument has nothing to do with this. The sine function has three zeros inside the circle C(0,6).

These are at 0 and
. If the circle had radius 7 instead we would need to consider the zeros at
as well.

2. I think that f is a hypothetical entire function such that

for all positive integers n.
Since it is entire it must be defined on the whole complex plane.

implies that
which clearly implies that
.
The point is that the poles of g do not lie in the open unit disk.

#61 Re: Help Me ! » combinatorics » 2009-05-19 09:38:10

I assume that by

you mean the chromatic number of
which I would normally denote
.

Suppose we have coloured H with d colours and we want to extend this to a colouring of G. Since G cannot be coloured with d colours we need to use a new colour for the vertex x. What does this tell you about the colours of the neighbours of x?

#63 Re: Puzzles and Games » Where Are The Cows? » 2009-05-12 09:25:27

quittyqat wrote:

It's impossible! To get to the goal,you have to choose "yes" to answer "Is the other marker in a box whose text refers to cows?".
There is NO such box!

I haven't solved this puzzle myself but I can see too possibilities.
1. If the other marker is also in box 50 then it is in a box whose text refers to cows.
2. If the rule change from box 60 is in effect then you always leave by the "yes" path.

#64 Re: Help Me ! » Harmonic Function » 2009-05-11 10:03:53

By definition, a harmonic function is a function whose Laplacian is 0.

Your proof that the sum of two harmonic functions is harmonic is fine, and is easily adapted to prove that the difference of two harmonic functions is harmonic.

Consider the function u(x,y) = x
u is harmonic but u^2 is not so the product of harmonic functions is not necessarily harmonic.
I'm sure you can find an example to show that the quotient of harmonic functions need not be harmonic.

#65 Re: Help Me ! » groups » 2009-04-08 11:28:36

Show that if


then
commutes with
.

Then, using that the group has odd order, it's not too hard to show that

is the identity.

#66 Re: Help Me ! » No idea where to start » 2009-04-02 10:13:00

Numerically Challenged wrote:

But I did forget to mention leading zeros are allowed

It doesn't matter. The clues define a linear system with eight equations and eight unknowns that has a unique solution. You don't need to know that the answers fit together in the grid, or that they are integers.

This is a very unusual cross-number puzzle.

#67 Re: Help Me ! » No idea where to start » 2009-03-31 10:33:23

I think you're right that starting and finishing aren't far apart.

#68 Re: Help Me ! » Quick question: alternating group » 2009-03-29 08:40:35

I would make the small complaint that the identity does not have one of the given forms.

In general, the problem is to find the partitions of n with an even number of even parts.

#69 Re: Help Me ! » Quick question on conjugate permutations. » 2009-03-29 08:36:06

Your question does make sense and you are right.

To prove that

are conjugate you must find
such that
.

You already have an element

such that
.

Suppose

. If you could find an element
that commutes with
then you might consider the element
.

If you are interested in a similar problem, you might like to think about the number of conjugacy classes of 5-cycles in

.

#73 Re: Help Me ! » Real Analysis Help » 2009-03-01 11:05:33

Jane, for any function


we have

so if your proof were correct then no such function could be continuous.
Clearly (0,1) is a closed subset of itself.

Here's a proof that works:

Suppose

is continuous bijection.
Let
and
.

If a < b then consider the set

.
Since [a,b] is connected, so is X. Also
so we must have X = [0,1].
This contradicts that f is a bijection.

Similarly if b < a.


PS: I feel pretty silly now. My proof is really just an application of the Intermediate Value Theorem.

#74 Re: Help Me ! » Real Analysis Help » 2009-02-16 09:58:39

The proof for question 1 that has been given is almost certainly the standard proof. However here is the idea of an another proof that I think is much prettier.

Suppose S is a non-empty set that is bounded below. Let L be the set of lower bounds for S. Show that L has a supremum and that infS = supL.

#75 Re: Help Me ! » Big-O and Big-Theta? » 2008-07-23 11:26:58

When the functions are polynomials, the most important thing to know is

In your first example we have


and so

I suspect that +1 changing to +n in your solution is a mistake.

If we want to show that


Then we could use the same idea

To get the answer that you have we need to observe that for x>9




and so

I prefer the first method, since it does not require one to think about the exact numbers involved.

I cannot give working for the Big Theta example since I do not believe that it is correct.

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