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## #1 Re: Help Me ! » Normalize » 2009-11-22 17:54:54

You got the right answer, but

must be real.

Note that

is on the unit circle when
are real, so it has absolute value 1.

Therefore,

Set

so
and
so
to get

Therefore,

and thus,

is normalized

## #2 Re: Help Me ! » Fibonacci's Sequence » 2009-11-16 06:52:39

Use mathematical induction

since

Assume

Then

so

since

shows it cannot be negative

Hence, by induction we have

for all positive integers n

## #3 Re: Help Me ! » Real analysis question involving closures and unions » 2009-11-11 15:45:09

Avon wrote:

The order of quantifiers is important.

betterthangauss wrote:

Therefore, for any neighborhood
of
, there is an integer
,
, such that

This is true but the value of
many depend of the neighbourhood
.

What you want to show is the there exists an integer
such that for any neighbourhood of
of

Thanks. After taking a second look at the problem I saw a really easy way to prove it. Namely:

Therefore,

Note that

is the finite union of closed sets, so it is closed (this doesn't hold in the countable case).

Therefore, since

is the smallest closed set containing
, we have

Thanks again for showing me my error.

## #4 Re: Help Me ! » Real analysis question involving closures and unions » 2009-11-10 22:23:01

Let me just post images instead, so that it's easier to read:

QUESTION

NOTE: By J_n I mean the set

## #5 Help Me ! » Real analysis question involving closures and unions » 2009-11-10 21:57:56

betterthangauss
Replies: 3

Let

be a finite collection of sets in a metric space
and set

Then the closure of

, namely
, satisfies

The union of the closures of A_i's being a subset of the closure of B is easy to show. However, the reverse direction is tripping me up a bit. Here's how I argued it:

Assume

Then

Then for any neighborhood
of
, we have

i.e., for any neighborhood

of
, we have

Therefore, for any neighborhood

of
, there is an integer
,
, such that

Hence, there is an integer

,
, such that
.

Therefore,

Hence,

Is this argument fallacious? I ask, because I can't see where the finite part of the union comes into play. It seems like I could use this exact same argument to say the result holds in the countable case too, but there is an obvious counterexample to that:

Namely, setting

to the set
of the rational numbers and each
to the singleton set
where
(since
is countable).

Then the closure of each

is just the set
, since each
is just the singleton
.

, the set of real numbers, while
.

Hence,  the two sides are not equal.

Can someone show me where this first argument above was going bad? Thanks.

## #6 Re: Coder's Corner » Best Language to Learn? » 2009-11-02 22:51:23

I like Scheme. It's a small language that allows one to quickly apply pretty profound ideas (such as procedures as data and vice versa). The greatest computer science book, scratch that, the greatest book I have ever read is the Structure and Interpretation of Computer Programs, which teaches how to program (using Scheme as its toolbox). Not only is this an incredible book; it's also 100% free from MIT Press' website.

http://mitpress.mit.edu/sicp/

In addition, you can watch video lectures from the actual authors of the book (and they're spectacularly done) at:

http://ocw.mit.edu/OcwWeb/Electrical-En … /index.htm

Not only that; you can also follow UC Berkeley's SICP class with prof Brian Harvey (another amazing teacher) at:

http://webcast.berkeley.edu/course_deta … 1906978389

If you like programming, this book and these sets of lectures are just loaded with mind-blowing moments. I challenge anyone to find a better and more creative computer science book.

## #7 Re: Help Me ! » LaTeX question about making command to write sets like {x | x^2 > 0} » 2009-11-01 10:08:03

Ricky wrote:

It's a good command to have, but with a pretty long name.  Reminds me of Java.  I would recommend:

SetC

Personally in commands, the more lowercase the better so I would actually make it

setc

When I'm coding something in any language, I only like to use short variable names for names defined in small scopes. For something global that's going to be used a lot to express the semantics of my paper, I'd much rather be as explicit as possible in my naming. I feel like if I stop writing a document and then come back a year later to make corrections or changes, I'll understand \SetWithConstraint more quickly than I will \SetC (maybe I'd think \SetC was a command for the set of complex numbers, for instance).  I use the camel-case with uppercase first letters to try to keep my defined names from conflicting with the symbols AMS defines.

## #8 Re: Help Me ! » LaTeX question about making command to write sets like {x | x^2 > 0} » 2009-11-01 00:31:14

math. Using \left to line up the | doesn't look right when the right side is small and the left side big

i.e., here's the code for the command:

\newcommand{\SetWithContraint}[2]{\left\{\,#1\ \left|\ #2\,\right.\right\}}

And if I use it like this:

$\Defined {B_N} {\SetWithContraint { \SetMember { \FiniteSum{a_i z^{n-i}}{i}{0}{n} } { \Polynomials{\ComplexNumbers}{z} } } { FOO! } }$

I get

where \SetMember, \Defined, etc. are commands I made up to try to decouple the logic of the markup from the characters used to  display it.

## #9 Re: Help Me ! » how to find sin 5 degrees » 2009-11-01 00:10:34

The following recursive algorithm will do it without any table lookup, and could be done by hand without having to do anything but multiply, subtract, and divide

It should be pretty fast, since dividing by 3 on each step will drop the argument to arbitrarily small size in logarithmic time. Of course, don't compute sin(x/3) twice in every iteration!

Here's a proof that it works:

## #10 Re: Help Me ! » LaTeX question about making command to write sets like {x | x^2 > 0} » 2009-11-01 00:09:10

Thanks. I completely forgot about being able to use the "." character in \left, \right commands as you suggested. It still doesn't look great like that, but it does match what I was asking for.. lol

Now the bar definitely looks too tall though, since it matched the size of the outer brace. I guess I should have asked if it's possible to make it the next size down from the size of the brace. Does anyone have a better idea for how to do this that doesn't have the limitations this command does?

\newcommand{\SetWithContraint}[2]{\left\{#1\,\left|\,#2\right.\right\}}

Is there a way to do this that wouldn't be considered cheating? Sorry if it's a noob question whose correct answer is "RTFM", but I haven't seen a good example of how to do this cleanly yet. Thanks again for the help, mathstyperson!

## #11 Help Me ! » LaTeX question about making command to write sets like {x | x^2 > 0} » 2009-10-31 23:09:04

betterthangauss
Replies: 5

I would like to make a command to write sets with constraints, so in my LaTeX I could write something like

\SetWithConstraint{x}{x^2 > 0}

Instead of having to manually do

\left{x | x^2 > 0 \right}

Here is the initial command definition I put in my preamble:

\newcommand{\SetWithContraint}[2]{\left\{#1\,|\,#2\right\}}

It seemed to work pretty well at first, for example this comes out looking pretty good:

However, I would like a way to have the size of the | in the middle match the size of the braces enclosing the set definition. My results here using that command look horrible:

Does anyone know a good way to accomplish this? I'd really prefer my TeX files weren't cluttered with garbage regarding explicit layouts of individual characters. Thanks!