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#251 Re: Help Me ! » Books » 2009-08-24 10:45:02

MIF, seems to be a forum bug, if I try to move the link "many others" up to the preceding paragraph, I get the error "BBC code cannot be nested in url tags".

#252 Re: Help Me ! » Books » 2009-08-24 10:43:43

Any (elementary) number theory, discrete math, or intro to proofs book will contain material on modular arithmetic.  If you want to just learn modular arithmetic, I would go with sources on the web.  Rutguers has an introduction, but there are

many others

#253 Re: Help Me ! » On the maximum of real-valued functions with complex image » 2009-08-24 10:37:06

Avon, we're are talking about functions that take on complex values.  Saying f(x) > 0 makes no sense.

does solving |g(k)|'=0 and g'(k)=0 give the same critical points?

Sorry, but once again I have to answer you with a question.

given a function f(k) defined on the reals

Does it map into the reals?  Or into the complex numbers?

#255 Re: Euler Avenue » To infinity and beyond. » 2009-08-23 04:16:17

George, you keep posting about this "element at infinity".  Prove it exists, or you have no claim to its existence.

"That which can be asserted without proof, can be dismissed without proof."

#256 Re: Help Me ! » On the maximum of real-valued functions with complex image » 2009-08-23 03:46:14

given a function f(k) defined on the reals and a complex constant z0, what is the maximum of following function?

You first need to phrase your question in a way that it is possible to answer.  Since z_0f(k) is of a complex value, there is no concept of maximum or minimum.  In fact, you can't even say one complex number is greater than another.  For example, which is greater, 1 or i?  You can talk about which modulus is greater (in which case, |1| = |i| = 1), and you can talk about when z_0f(k) achieves its maximum modulus.  Is this what you want?

#257 Re: Help Me ! » Books » 2009-08-23 03:37:03

Dummit and Foote have a great text for Abstract Algebra.  Durin is easier to follow, but also no where near as in depth as Dummit and Foote.  Many people recommend Munkres for topology, but I haven't worked out of it.  Calculus I, II, and III are covered by Stewart, this is what I learned from and it was fine.  It also covers some analytic geometry (as all multivariable texts do), but not in depth.

#258 Re: Help Me ! » Four people travel...why? » 2009-08-21 02:34:13

Your solution appears to be correct.

#259 Re: Help Me ! » Concave Up Down, HELP » 2009-08-18 11:07:00

A function is increasing if f'(x) > 0.  It is decreasing if f'(x) < 0.  In order to find the intervals where f'(x) is increasing, we first look for all the points where:

(i) f'(x) = 0

OR

(ii) f'(x) has a discontinuity

Note that it is very possible for f'(x) to exist and have a discontinuity, but certainly any place where f'(x) does not exist will also be a discontinuity.  Once you've found these points, they will be the intervals where f'(x) is either greater than or less than 0.  Now all you need to do is test a point in each one of these.

Concavity is exactly the same except we're now using f''(x).

#260 Re: Help Me ! » What do we get from log (-1)? » 2009-08-18 00:32:53

Thanks for the answers - so it seems if one tried to define some "new maths" involving ln(-1) it would turn out that sqrt(-1) could be defined using the "new" value ln(-1).

That is precisely correct.  In fact, in complex analysis we can define a square root if and only if we can define a log.

Where l(z) is a branch of the log.  Intuitively

But there are problems with just saying "z^1/2".

#261 Re: Help Me ! » Roots And Matrices » 2009-08-18 00:27:33

What I mean to say is I dont have the patitence to read wikipedia(lol!)

Then why do you expect to have the patience to read whatever it is we write?  This site is mostly for answering questions, not replacing your textbook.

#262 Re: This is Cool » Chess – Win – Draw – Lose ? » 2009-08-18 00:23:19

MathGuise was Anthony R. Brown, again harassing the forums.

#263 Re: Help Me ! » Limit » 2009-08-14 12:57:13

lashko wrote:

No L'Hopital does not apply. Ive thought about it

Huh?  The most you can say is "I don't see how to apply it".


#264 Re: Help Me ! » how to solve this? » 2009-08-13 18:06:10

I have a deductive argument proving that there is only one solution (and finding it).  Unfortunately it's about 6 pages or so (but I think it can be shortened quite a bit).  It was actually quite fun, Sudoku like.

#265 Re: Help Me ! » how to solve this? » 2009-08-13 15:09:47

That is the only solution.

For[a2 = 0, a2 <= 6, a2++,
  For[a3 = 0, a3 <= 2, a3++,
    For[a5 = 0, a5 <= 6, a5++,
      For[b2 = 0, b2 <= 6 - a2, b2++,
        For[b3 = 0, b3 <= 2 - a3, b3++,
          For[b5 = 0, b5 <= 6 - a5, b5++,
            For[c2 = 0, c2 <= 6 - a2 - b2, c2++,
              For[c3 = 0, c3 <= 2 - a3 - b3, c3++,
                For[c5 = 0, c5 <= 6 - a5 - b5, c5++,
                  d2 = 6 - a2 - b2 - c2;
                  d3 = 2 - a3 - b3 - c3;
                  d5 = 6 - a5 - b5 - c5;
                  
                  If[2^a2*3^a3*5^a5 + 2^b2*3^b3*5^b5 + 
                    2^c2*3^c3*5^c5 + 79*2^d2*3^d3*5^d5 == 711, 
                   Print[{a2, a3, a5, b2, b3, b5, c2, c3, c5}]];
                  ];
                ];
              ];
            ];
          ];
        ];
      ];
    ];
  ];

#266 Re: Help Me ! » Limit » 2009-08-13 13:34:30

L'Hopital is the way to go.   Do some algebraic manipulation till you get it into this form:

And note that the limit of the right factor is zero.  Now L'Hopital, and then a little bit more algebraic manipulation and you're done.

The limit actually turns out to be about

#267 Re: Help Me ! » proof of the sum of squares » 2009-08-13 10:46:31

Lashko, just use the fact that:

#268 Re: Help Me ! » When can this be simplified? » 2009-08-12 12:53:11

Just because the limit of f(x) as x approaches c exists does not mean that f(c) is equal to this limit.

#269 Re: Help Me ! » nice fact » 2009-08-11 09:35:02

farah345 wrote:

The answer will always be 9. Can someone prove how?

1111-1111 = 0

#270 Re: Help Me ! » Proof of Corollary » 2009-08-09 04:30:54

A corrollary should follow from a preceeding theorem, so that's the first place to look.  What book/page number is this?

I'm guessing that L_n is an nth order differential equation?  That determinant is almost exactly the same as the Wronskian, I would first calculate that out.  You should be able to come up with a pretty simple closed form for it.

#271 Jokes » World's worst dictionary » 2009-08-06 10:36:22

Ricky
Replies: 2

Word: Definition

1. Recondite: Abstruse
2. Abstruse: Enigmatic
3. Enigmatic: Cabalistic
4. Cabalistic: Arcane
5. Arcane: Esoteric
6. Esoteric: Hermetic
7. Hermetic: Recondite

(This all started when I saw the definition of recondite including the word "abstruse")

#272 Re: This is Cool » The 46656 Math Problem. » 2009-08-06 03:41:51

There was a game like this, I believe it was called "Black box".  The only difference is that it would tell you how many you got right (i.e. in the right place as well), and how many numbers you guessed but were in the wrong place.

#273 Re: Help Me ! » Limit » 2009-08-05 18:44:33

Unfortunately soroban, the question was to prove that it converges to 0, not just to show convergence.

#274 Re: Help Me ! » Derivative of a matrix » 2009-08-05 13:44:07

Huh, weird, but I guess it's ok.  I've never heard of derivatives talk about in this way.

Just write the entire thing out.  Fix an i and j, and in terms of a_ij, what is A(t) - A(gamma)?  And then what is this when it is divided by t - gamma?

#275 Re: Help Me ! » Derivative of a matrix » 2009-08-05 12:41:54

What is X?

It looks to me like you're trying to define the derivative of a n to m dimensional function, but this doesn't make sense because you say that I is an interval.  So maybe you are attempting to define the derivative of a path?

Either way, please remember that there is no such thing as a derivative of a matrix.  A derivative of a function can be represented by a matrix, but this is still a derivative of a function.

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