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#26 Re: Help Me ! » Epsilon Delta Proof » 2009-11-03 09:23:44


First observe that for all
we have

We know we are going to be able to bound

so the important thing is to bound x away from 0.

If

then
and so

For any

we have:
if
then

Hence if

then
whenever

#27 Re: This is Cool » Equal positive integers » 2009-10-07 08:34:16

bobbym wrote:
Bladito wrote:

Assume that the theorem is true for some value k.

One problem is right there, you can't assume that. You can't find any k where that is true to start the inductive process.

The statement is true when k=1 as Bladito has stated. You seem to agree.


Bladito, the problem is that A-1 and B-1 are not necessarily both positive integers.

#28 Re: Help Me ! » Remainder Theorem [Hard] » 2009-10-05 22:15:30

Hi bobbym

My main complaint was that you were assuming that P(x) was linear.
There is no harm in assuming that the remainder is linear, since it must have degree less that (x+2)(x-1).

Hence you could start by writing P(x) = (x+2)(x-1)Q(x) + ax + b  for some polynomial Q(x)
and then prove that a = 7 and b = -5 as you have done.

It probably is a little neater to do it that way than the way that I did it.

#29 Re: Help Me ! » Remainder Theorem [Hard] » 2009-10-04 22:44:03

You really shouldn't assume that P(x) is a linear polynomial.
P(x) = x^2 + 8x - 7 is another polynomial with the required properties.

We know that when P(x) is divided by (x+2) the remainder in -19.
This means that P(x) = (x+2)Q(x) - 19 for some polynomial Q(x).

We also know that when P(x) is divided by (x-1) the remainder is 2.
As you observe this means that P(1) = 2.
So 2 = P(1) = (1+2)Q(1) - 19 so 3Q(1) = 21 and hence Q(1) = 7.
This means that Q(x) = (x-1)R(x) + 7 for some polynomial R(x).

Therefore P(x) = (x+2)[(x-1)R(x) + 7] - 19 = (x+2)(x-1)R(x) +7(x+2) - 19
so P(x) = (x+2)(x-1)R(x) + 7x - 5.

The remainder when P(x) is divided by (x+2)(x-1) is 7x - 5.

#30 Re: Help Me ! » trace is a non-singular » 2009-10-04 13:47:42

The trace map is a polynomial over the field F_{q^n}. How many roots can it have?

#31 Re: Help Me ! » Sylow subgroups of S5 and S6 » 2009-10-04 08:51:46

Sylow's theorems tell you that any two Sylow p-subgroups of a group G are conjugate in G so if you can find one it shouldn't be too hard to find them all. (I assume that "all" in quotation marks is acknowledging this.)

Sylow's theorems also tell you that any p-subgroup of G is contained in a Sylow p-subgroup of G, so in these small cases it shouldn't be too hard to find a p-subgroup and if it is too small find a bigger one containing it.

Can you determine the orders of these Sylow p-subgroups?
For instance a Sylow 5-subgroup of S6 must have order 5. Can you find a subgroup of order 5 in S6?
Finding a Sylow 3-subgroup of S6 isn't too much harder.

To find a Sylow 2-subgroup of S5 consider what its order has to be, and also what the order of a Sylow 2-subgroup of S4 must be.
We can naturally embed S4 in S5 so any Sylow 2-subgroup of S4 must be contained in a Sylow 2-subgroup of S5.

If you have found a Sylow 2-subgroup of S5, then you want to find a Sylow 2-subgroup of S6 containing it.
Consider what the order of such a subgroup should be and I don't think it is too hard to find.

#32 Re: Help Me ! » Answer different from book » 2009-09-26 17:39:33

The two answers are equivalent.

At any point on the curve defined by


we have

#33 Re: Help Me ! » infinite series » 2009-09-24 09:59:48

Since you can differentiate a power series term by term within the radius of convergence the following are all valid when |x| < 1

and your result follows with x = 1/2

#34 Re: Help Me ! » non-abelian groups of order 27 » 2009-09-23 09:59:06

Ricky wrote:

First prove that no element can be of order 9.

Z9 has an automorphism with order 3, so there is a semi-direct product of Z9 and Z3 that is non-abelian.


To answer the second part you should consider the subgroups of G/Z(G).

#35 Re: Help Me ! » non-abelian groups of order 27 » 2009-09-22 11:30:10

I don't think Sylow's theorems can be used to solve these problems.

Some results that I think are more useful are:
If G/Z(G) is cyclic then G is abelian.
If G is a finite p-group then Z(G) is non-trivial.
If G is a p-group and H is a subgroup of index p in G then H is a normal subgroup of G.

I assume that part 2 is supposed to be "How many subgroups of order 9 are there?".

#36 Re: Help Me ! » On the maximum of real-valued functions with complex image » 2009-08-24 10:10:59


Suppose
is a continuous function and let

If

is differentiable at
and
then
on some neighbourhood of
and so

If
is differentiable at
and
then
on some neighbourhood of
and so

Hence if

is differentiable at
and
then
is also differentiable at
and
if and only if

It follows that a critical point of

must be either a critical point of
or a zero of

If the maximum of

is attained at a zero of
then we must have that
is identically zero, and so every point is a critical point of

Therefore if

has a maximum value, it is attained at a critical point of

#37 Re: Puzzles and Games » Edna's age » 2009-08-19 09:48:10

I like this kind of puzzle. I find that the best way to make any sense of sentences like these is to read them from right to left.

#38 Re: Help Me ! » Modulo Help » 2009-08-16 09:58:05


Note that the linked document says
and not 0.25.

Since 4*6 mod 23 = 1 we have

#39 Re: Help Me ! » How do i work out an average? » 2009-07-20 09:48:53

Ricky wrote:

Anyone have the faintest idea why this thread has over 35,000 views?

This thread is the first page that comes up when I search for its title (or even just 'work out average') on Google.

#40 Re: Help Me ! » simplify to cosh(x) » 2009-07-19 11:20:55

bobbym,

soroban's antiderivative is


rather than

Since


it follows that

and so

#41 Re: Puzzles and Games » relatively prime » 2009-07-17 16:14:50

Since a and b are coprime we must have

and so

and also


Since a and b are positive it follows that

and

and so

Hence
so

Since
we have


Now


and

Therefore

#42 Re: Help Me ! » LOG question » 2009-07-14 11:03:10

Assuming that "underoot" means "square root"

#43 Re: Help Me ! » Real Analysis Help » 2009-06-24 10:24:59

jay17 wrote:

x^4 - 10x² + 1 = 0

you can see that the only rational roots are +- 1,  contradiction

I would interpret this to mean that 1 and -1 are roots of this polynomial, which is clearly untrue.

It would be better to say something like, by the rational root theorem any rational root of this polynomial is either 1 or -1.

#44 Re: This is Cool » Mentalism? » 2009-06-23 14:57:31

I will admit that my choice of 610, 987 was rather extreme. I certainly wouldn't expect this pair to be found by randomly trying pairs of integers, but even 1, 2 is a better starting pair than 1, 1.

The important thing is that the ratios of consecutive pairs of Fibonacci numbers are the best rational approximations to the golden ratio.

#45 Re: This is Cool » Mentalism? » 2009-06-23 12:18:29

MathsIsFun wrote:

The convergence seems to work best if the two positive integers are the same, and worst if the second is zero

What do you mean by "work best"?

I can't see a measure of goodness of convergence such that both
i) Starting with 1, 1 works better than starting with 1, 0
ii) Starting with 1, 1 works better than starting with 610, 987

#46 Re: Help Me ! » calculate θ angle » 2009-06-23 10:37:39

Since


the arctangent of y/x does not completely determine the angle.
For this reason, programming languages usually provide the atan2 function.

#47 Re: This is Cool » Mentalism? » 2009-06-23 10:18:44

The trick doesn't quite work for any two starting numbers. Let


and choose
and
as the starting numbers.

Since

the sequence we get is

so the ratio of two consecutive terms is always

#48 Re: Help Me ! » help needed » 2009-06-17 10:31:19

Jane: Using
as a substitute for
is just awful.

#49 Re: Help Me ! » 10^n + 1 » 2009-06-17 10:18:18


It follows from the result I prove in this post that if
is prime then n is a power of 2.

I got my computer to test if

is prime for
and the only primes it found were 11 and 101. I don't have enough memory to make k any higher with my brute force algorithm.

There is a conjecture that

is prime for only finitely many n, an extension of the conjecture that only finitely many Fermat numbers are prime.

#50 Re: This is Cool » Prime Numbers!!! » 2009-06-14 23:27:56

It is telling me that 1111111111111111111 is prime. However, the 1 in [ <1111111111111111111, 1> ] means that the prime factor 1111111111111111111 has multiplicity 1.
For instance, Factorization(20); will output [ <2, 2>, <5, 1> ].

I'm still unsure what RickIsAnIdiot means by "shows all the digits".

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