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#126 Re: Help Me ! » I've found this to be INTERESTING.. » 2010-02-20 04:35:15

That's why I clearly said for small mod.

No, you said "Computing has replaced thinking."

#127 Re: Help Me ! » Polynomial Reconstruction in Finite Field » 2010-02-20 04:30:27

My question: How can I re-create the above polynomial using the finite field coordinate locations?

You can't.

First off, four points determine a cubic polynomial in the real numbers.  So consider the polynomial which passes through

(0, 4 + 10*251), (1, 163 + 251), (2, 1888 + 5*251), and (3, 6685 * 251)

And remember that 4 points uniquely determine a cubic.  This cubic must be different than the one you wrote, and so these points are not enough to determine your polynomial.

#128 Re: Help Me ! » Topology: Covers of figure-eight space » 2010-02-18 14:14:30

I'm counting 6, and I believe my enumeration covers all cases (up to equivalence).

#129 Re: Help Me ! » I've found this to be INTERESTING.. » 2010-02-18 14:00:24

Computing has replaced thinking.

Let's see you go compute all groups of order 8633 up to isomorphism.

Principle of Expediency: A good answer today is better than a great answer tomorrow.

Why are both not an option?

In any case, I'll wager the time it took for me to find those results was much shorter than the time it would take you to write a program.

#130 Re: Help Me ! » I've found this to be INTERESTING.. » 2010-02-18 09:35:41

Normally for a small mod like that I would just plow through the 64 possibilities of

My post reduced it to 15 possibilities.

#131 Re: Help Me ! » I've found this to be INTERESTING.. » 2010-02-18 02:08:47

First: all x,y that are divisible by 9 are solutions.

This is what ZHero meant by (0,0).  Anything congruent to a solution will of course be a solution.

ZHero, the way you typically do these problems is you eliminate groups of cases till there is a small enough number to check by hand.  For example, (x, 0) and (0, y) will not be solutions for x and y nonzero.  Further, there are no solutions for x=1 or y=1.

No solutions where x+y = 9 except for x=3 and y=6 (or vice versa), and this is a solution.

#132 Re: Help Me ! » Topology: Covers of figure-eight space » 2010-02-16 14:41:45

It sounds like you don't have many tools, for example the correspondence between covering spaces and subgroups of the fundamental group.  If that's the case, then I don't know how you'd ever prove you have all.  Think about the behavior at the center point of the figure 8.  In a neighborhood around the preimage of this point, your cover must at the same way.  All other points are uninteresting because lines will lift to lines (remember, you're looking locally).

#133 Re: Help Me ! » Combinatorics » 2010-02-16 10:41:28

Break it down.  First solve the following problem:

If z and w are any non-negative integers, how many solutions are there to:

z + w = n

Where n is any integer.

Once you know this, we can use this solution because we just need to the number of solutions to:

z + w = 20 - x - y (= n)

So we can split it up into cases for each pair (x,y) with x > y.

#135 Re: Help Me ! » Limits !!!! » 2010-02-15 12:50:11

What definition?  The epsilon-delta definition?

Number 2 is wrong.  Should that be x approaching infinity?

#136 Re: Help Me ! » Uhh... ridiculously easy D1 question or not? » 2010-02-13 08:55:38

If so, jeez... whats the point o.o

To give 1 point to people who at least understand the question.

#138 Re: Help Me ! » different sizes of infinity » 2010-02-10 09:25:39

Before you start considering sequences, it is best to say what we mean by the "size of infinity".  Of course when we say this, we mean the size of an infinite set.

So we can begin with something you know: the size of a finite set.  For example: {a, b, c} and {1, 2, 3}.  These sets have the same size.  You could say that their size is three, but the genius Cantor discovered that you don't need numbers to talk about the relative sizes of these sets.  We say that two sets are the same size if you can pair elements together so that:

(a) You don't use a single element twice and (b) every element is used.

For example, a pairing from the sets above is: {a, 1}, {b, 2}, {c, 3}

It should be obvious that if two finite sets have the same size, then you can pair their elements in this way.  And if course, if two finite sets can be paired, then they have the same size.

Now here is the kicker: When it comes to infinite sets, we can't describe their sizes by a number.  But we can still try to pair elements.  For mathsyperson's example, we have the natural numbers and the even natural numbers.  Here is a pairing:

{1, 2}, {2, 4}, {3, 6}, {4, 8}, {5, 10}, ..., {n, 2n}, ...

I use every element from each set only once, and I used every single element from each set.  Therefore, the natural numbers have the same size as the even natural numbers.  Now this may seem odd, because the even numbers are properly contained inside the natural numbers, but infinity is odd, so that's ok.

Cantor proved that you can't pair the integers with the real numbers.  No such pairing can exist.  This is known as Cantor's Diagonal Argument.  This is why we say the real numbers are bigger than the integers, there is absolutely no way to pair them.

#139 Re: Help Me ! » Discontinuous Derivative » 2010-02-10 09:14:10

It is a piece-wise definition.  I was being a bit informal before, so here is the actual definition:

#140 Re: Help Me ! » Discontinuous Derivative » 2010-02-08 09:50:17

The function

Is differentiable at the origin, but it's derivative is not continuous there.

#141 Re: Help Me ! » Show that: A∆(B∆C) = (A∆B) ∆C » 2010-02-08 03:04:36

Δ here is symmetric difference of sets?

#142 Re: Help Me ! » one group problem » 2010-02-08 03:03:10

Use Bézout's identity to prove the proposition.

#143 Re: Help Me ! » Help !! Find all the functions ! » 2010-02-06 07:08:37

All functions?  They need not be continuous?

There are two points you should start with, and they aren't hard to find.  What are they?

#144 Re: Help Me ! » one group problem » 2010-02-04 11:11:05


What I find a bit interesting is that the module structure of an abelian group doesn't add any information, but makes the above proposition more straightforward.

#145 Re: Help Me ! » Solve The System » 2010-01-31 15:22:53

y must be in the set {-3, -2, -1, 0, 1}

It should be easy from here.  To come up with this restriction, use my previous post #6.

#146 Re: Help Me ! » Solve The System » 2010-01-31 15:16:42

When divided by 3, y can not leave a remainder of 2.

#147 Re: Help Me ! » Solve The System » 2010-01-31 15:14:04

Substitute x = 1-y-z into the 2nd equation, and you get

This gives the infinitely many solutions for z=1, and y can be chosen arbitrarily with x = 1-y-z.  It also proves that if z is not 1, then 3 must divide z.

#148 Re: Help Me ! » Solve The System » 2010-01-31 12:54:06

Do you need all solutions?  There is an obvious one you can see.

#149 Re: Help Me ! » Find all the functions !! » 2010-01-31 06:36:00

What is f(1)?  What about f(3)?

#150 Re: This is Cool » "Factor Space" » 2010-01-27 18:04:16

This is not quite a useful invariant, but it may illustrate the purpose that invariants serve.

Let A and B be two finite sets of integers.  Then each set has an invariant: odd or even, if the sum of their numbers is either odd or even.  As with all invariants, this can tell you when two sets are not the same: a set which adds up to an even number can never be the same as a set which adds up to an odd number.  Of course it can not tell you when two sets are the same as 2 + 8 = 6 + 4.

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