# John

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## Posts posted by John

### Converse of alternative series test?

Hm.

I think

$a_n = \frac{\sin{n} + 1}{n}$

works. WolframAlpha seems to think it does, anyway.

### Big O notation for a small code segment

c+c*N(N+2) + c

And since we are only concerned with N:

N(N+2) = N^2 + 2N

O(N^2)

So, this is O(n^2).

I'd say O(n). The innermost loop only iterates through four values, since k starts at N - 2.

### Is this Njwildberger guy (in YouTube) any good?

Well, I wouldn't necessarily say he's a crank, but he is an ultrafinitist, so while his videos may be decent overall (and I've heard they are), bear in mind that he's philosophically opposed to infinite sets, irrational numbers, and even finite numbers large enough that they cannot be physically represented. This goes against the mainstream foundations of modern mathematics. He's also attempted to reformulate trigonometry into rational trigonometry, which is very much fringe mathematics.

As an example of his position, a few articles expressing his views can be found at his page here: http://web.maths.unsw.edu.au/~norman/views.htm

I don't know how much all this will factor into any videos he's done on geometry, but it's something to look out for.

### Converse of alternative series test?

What you're asking about is the inverse, not the converse, and the inverse is not necessarily true. For instance, consider

$\sum_{n = 1}^{\infty} (-1)^n a_n$

where

$a_n = \begin{cases} \frac{1}{n^2} & n \textnormal{ is odd} \\ \frac{1}{n^3} & n \textnormal{ is even}. \end{cases}$

Then our series is

$\sum_{n = 1}^{\infty} (-1)^n a_n = -1 + \frac{1}{8} - \frac{1}{9} + \frac{1}{64} - \frac{1}{25} + \frac{1}{216} - \frac{1}{49} + \cdots$

which converges even though $a_n$ is not monotonically decreasing.

The converse of the alternating series test would be that if the series is convergent, then the absolute values of the terms are monotonically decreasing and the limit is zero. Since the converse is the contrapositive of the inverse, we see that the converse is not necessarily true either.

### inequality

But it's clear what the relationship must be. We must have $\varepsilon < \min(r, b - a)$. If you want an equality, use the fact that $\min(x, y) = \frac{x + y - |x - y|}{2}$.

### Supporting yourself through school.

In the U.S., there are loads of different available scholarships and grants, though of course there's no guarantee of getting them since they're mostly competitive or tied to certain qualities not every student will have. The government offers Stafford loans, which are granted regardless of financial need, though whether interest accumulates during your degree does depend on whether you have financial need, there is a maximum amount you can take out each year and overall, and how much you can actually take out each year is tied to the cost of attendance for your school. There are also Pell grants available from the government, though they're based on financial need.

Individual states may also offer some assistance. For instance, I'm from Louisiana, and there's a program there called TOPS, which pays full tuition and fees for any public school in Louisiana for four years as long as you take certain classes in high school and achieve a certain GPA and ACT score (and assuming you attend college shortly after high school).

Other than that, it's similar to what hyper mentioned above. People go to school while working. Given the odd hours that taking classes sometimes imposes on availability for work, some people wind up working multiple jobs. It's difficult, especially in a degree like physics, but it's doable. Of course, there are also online degrees available in some subjects, or at least individual online courses available. The perception of online classes/degrees varies (as an example, the medical school in the city I come from won't accept online classes for any of its specific prerequisites, even the non-science ones). Another option is to prioritize some job and take classes during non-business hours, though this may entail taking far longer than four years to complete a degree. Depending on the job and possibly the degree, an employer may even offer some financial assistance.

C has a very simple syntax, and those seven statements constitute pretty much all there is in terms of controlling the execution of code. Other than that, it's a matter of becoming familiar with the various data types, operations we can perform on those data types, knowledge of the functions built into the standard library, and construction of new functions. There are also sometimes shortcuts for certain statements, e.g. in C, we have the ternary operator ?:, used like this:

condition ? statement1 : statement2;

which can take the place of

if(condition) {
statement1;
} else {
statement2;
}

Other languages (like Python) provide more shortcuts to performing various actions, more built-in functions, etc. These collectively are referred to as "syntactic sugar." For instance, the Hello World program in C is

#include <stdio.h>

int main(void) {
printf("Hello world.\n");
return 0;
}

while in Python it's

print("Hello world.")

So the number of words required to write a program depends on the language. Still, I'd say the seven control statements I listed in my previous post are enough to at least express decent programs in pseudocode.

I'm not sure exactly what you're asking, but in terms of program control, if we look at C, we have

goto (as noted) to immediately execute some labeled statement,

if/else to make a decision based on some condition,

switch to enumerate a variety of outcomes based on the value of some variable (similar to an "if/else if/else if/.../else" statement),

for, while, do/while to loop through the same block of code as long as some condition is met,

break to immediately exit a loop and move on to the first statement after the loop,

continue to immediately force another iteration of a loop without executing further code within the loop, and

return to allow a function to pass some result back to whichever function called it.

### Do numbers actually exist?

We can also induce a partial order on the complex numbers using the concentric circles of magnitude mentioned earlier, counting clockwise around each circle before moving to the "next." It's probably easier to think about this in terms of polar coordinates, in which we can say that $(|z_1|, \theta_1) < (|z_2|, \theta_2) \iff \left(|z_1| < |z_2|\right) \textnormal{ or } \left(|z_1| = |z_2| \textnormal{ and } \theta_1 < \theta_2\right)$. However, this is a bit unsatisfying since it means, among other things, that 1 < -1.

Just noticed this should say "counterclockwise" rather than "clockwise." We could go clockwise too, I suppose, but then we'd need $(|z_1|, \theta_1) < (|z_2|, \theta_2) \iff \left(|z_1| < |z_2|\right) \textnormal{ or } \left(|z_1| = |z_2| \textnormal{ and } \theta_1 > \theta_2\right)$, since angles increase in the counterclockwise direction.

I give up. I've never evaluated inequalities between vectors.

Well, in this case, the inequalities are just between magnitudes and angles, both of which are scalar values.

### Do numbers actually exist?

There is an imaginary solution mathematically, but what I was referring to was the physical interpretation. That is to say, physically, there is no time at which the ball is at a height of four meters so long as it's following the parabolic trajectory defined by -t2 + 3 = h. We mustn't confuse the mathematical model of a system with the system itself.

As for applications of complex numbers in physics, I'll have to leave that for those who know more about physics and applied mathematics. My current knowledge there doesn't extend much beyond knowing that there are applications.

### Do numbers actually exist?

Alright, I see what you're saying. The situation then is that you're standing at a height of three meters, dropping a bowling ball, and then trying to determine at what time the bowling ball will be at a height of four meters. But a height of four meters doesn't lie on this parabola, so there's no solution. Of course, in practice, you could drop the bowling ball onto a machine that will catch the ball and launch it with enough force to send the ball four meters or higher into the air, but then, mathematically, you're dealing with a different parabola.

Edit: Just randomly decided to search for "imaginary time" and this came up: http://en.wikipedia.org/wiki/Imaginary_time. But it's something you'd probably want to discuss in the Physics forum.

### Do numbers actually exist?

Hmm, I think I see where the ambiguity lies. If the imaginary component is bigger than that of another, but the real component is smaller than that of the same other, you cannot determine which is fundamentally greater?

Well, we can always just go by the magnitude to get a sense of relative sizes. We just have to accept that in that context, certain apparently unequal numbers are equal in magnitude, e.g. 1 has the same magnitude as i and -1. The difficulty arises when we try to put the complex numbers into some order such that two unequal numbers have unequal positions in the order, e.g. even though 1 and i have the same magnitude, if we want to order the complex numbers, we'd like either i < 1 or 1 < i (but not both), and fulfilling either requirement can be tricky, especially once we start adding operations into the mix.

For practical purposes, lets say I'm dropping a bowling ball from a height of 3 meters, but I solve for 4 meters of a downward parabola with a maximum. Where is that imaginary solution in physical reality? Where is that extra imaginary dimension I'm not seeing?

Hm, I'm not sure what you're asking here. The parabola has real solutions for all t, but in practice the Earth will keep the bowling ball from following the parabola past 3 meters, so in practice (disregarding wind and other such adjustments we'd need to make), the bowling ball will follow the parabola until it bounces, then follow another until it bounces again, and so on until it comes to rest.

The parabola only describes the ball's path from the moment you throw/drop it until the moment it collides with some other object, so at the risk of upsetting any philosophers of science lurking around, the fact that the graph of the parabola continues down past h0 - 3 meters doesn't have a particularly profound meaning.

Edit: To answer your question, the solution is real not imaginary, and it lies one meter under the ground.

### Do numbers actually exist?

I had to look up John's terms, and I'm still uncertain. Maybe he can clarify it for the both of us.

I'll try. Just let me know which part(s) need clarification beyond what Wikipedia can provide.

I see more clearly where the infinite numbers are coming from, since imaginary numbers in a way act like their own dimension that also acts to treat fundamental operations differently, whereas just real numbers on a number line would not create a circle of possibilities. Is there any manner of expressing that one complex number is greater than or less than another at all?

Yes. We can induce a total order on the complex numbers by moving up along the imaginary axis and then right along the real axis, i.e. $a + bi < c + di \iff [(a < c) \textnormal{ or } (a = c \textnormal{ and } b < d)]$. So for example, i < 100i < 1 < (2 - 100i). It's an odd sort of ordering if we're looking at the numbers in terms of their conceptual magnitude, but as far as I can tell it does satisfy the axioms.

We can also induce a partial order on the complex numbers using the concentric circles of magnitude mentioned earlier, counting clockwise around each circle before moving to the "next." It's probably easier to think about this in terms of polar coordinates, in which we can say that $(|z_1|, \theta_1) < (|z_2|, \theta_2) \iff \left(|z_1| < |z_2|\right) \textnormal{ or } \left(|z_1| = |z_2| \textnormal{ and } \theta_1 < \theta_2\right)$. However, this is a bit unsatisfying since it means, among other things, that 1 < -1.

### Do numbers actually exist?

We need to be careful with our terminology here. The standard order we use with the real numbers is not a well-order, since a well-order requires that any non-empty subset has a least element, which is not true of the reals (consider, for example, the interval (0, 1)). It is, however, a total order.

If we assume the axiom of choice, then we have the (logically equivalent) well-ordering theorem, which states that any set can be well-ordered. So while the standard order on the reals is not a well-order, if we assume the axiom of choice, then there does exist a well-order for the reals, as well as a well-order for the complex numbers. Whether the order is meaningful, much less useful, is another story.

However, what we can't achieve with the complex numbers is an ordered field structure, since the square of any non-zero element of an ordered field must be positive, but we have i2 = -1.

Edit: If we try to order the complex numbers simply according to their magnitude, then we find that infinitely many complex numbers share any given magnitude. Looking at the complex plane, what we see is each magnitude |z| represented by a circle of radius |z|. This is what studiot was saying. So for instance, looking strictly at magnitude, -1 = i = 1 = -i. What we can do in that case is define the equivalence relation ~ such that z1 ~ z2 if |z1| = |z2|. But all we accomplish with that is mapping each equivalence class to a real number, thus it's not exactly a mathematical breakthrough and doesn't really accomplish what we want.

### MMORPG about educating people on how hacking works

Er, DDoS stands for "Distributed Denial of Service." It has nothing to do with DOS the (family of) "Disk Operating System(s)."

In any case, yes, it's very easy to launch a DoS attack, and even to write a special program for it. That was my entire point, that being able to DDoS a server requires very little technical know-how, yet attacks on that level are still often referred to as "hacking" by the media.

Anyway, as mentioned, it's a bit off-topic, so I won't be saying more about it in this thread. I was simply elaborating on pavel's point in his first post.

### MMORPG about educating people on how hacking works

I'm not sure what you're getting at. LOIC has certainly been used in several high-profile DDoS attacks in recent years (for example, some discussion and links to related news stories can be found here: https://www.us-cert.gov/ncas/alerts/TA12-024A).The JavaScript implementation is interesting, but it doesn't support the entire feature set of LOIC itself (which is a Windows desktop application written in C#).

In any case, that's a tangent of a tangent.

Regarding the actual thread topic, as others have mentioned or alluded to, striking the proper balance between authenticity and entertainment is tricky. The subject in general is probably also a bit esoteric if you're looking for something with mainstream appeal. Then again, 0x10c faced similar issues, and there was some excitement building around it, though it did have the benefit of Markus Persson's name being attached to it. Of course, 0x10c has been put on indefinite hold, but apparently that's due to notch being unsure of where to go with it.

### MMORPG about educating people on how hacking works

The game might be fun, but if it were at all realistic, it would probably have limited appeal. However, if you're learning game design or software engineering generally, it might be a fun side project regardless of its ultimate success. I would have said something similar about Minecraft before its release, though, so eh. You never know what will catch on.

As for the term "hacker," pavel was referring to the original meaning detailed here. Of course, particularly clever methods found for breaking security might justifiably be called hacks, but nowadays any 15-year-old who can work out how to install LOIC might be called a "hacker" by the media (including tech-oriented media) as well as by computing enthusiasts in general. I think that particular battle has been lost, anyway, so it's a minor point.

### Zero = Nothing?

Now, we have things called imaginary numbers. The sqrt(-1) is also NOT a number.

Is sqrt(-1) is undefined? I think sqrt(-1) is undefined on the set of real numbers. Just like the x in x = x + 1.

So, the 'undefined in terms of real numbers' nature of some mathematical entities doesn't mean we can't make sense of it in a different, perhaps useful, way.

Well, as mentioned, there are other structures in which we can assign a solution. But the spirit of the OP seems to be that we want to be able to use x with standard operations in standard arithmetical expressions--that is, in some structure similar to or extending the real numbers, and that's a bit tricky.

Imaginary numbers are certainly numbers, and the choice of the term "imaginary" is a bit unfortunate. Complex analysis is one of the most useful branches of mathematics, both in mathematics and elsewhere.

We can go even further, into systems like the quaternions and even the octonions, where we have multiple square roots of -1. While we do lose the field structure in these more exotic systems, they still find application in various branches of mathematics and physics.

### Zero = Nothing?

1. What we mean by that is that in the field of real numbers (that is, the set of all real numbers along with addition and multiplication defined in the usual way), there is no x such that x = x + 1. It's fine syntactically and (I would argue) semantically, but there is no solution to the equation.

2. Infinity isn't undefined, but it also isn't a real number. Thus, as mentioned by studiot earlier, we'd need to move to a structure in which infinity is an element and define addition such that infinity + 1 = infinity. But we still have to be careful about how we operate with infinity.

3. I suppose so. Before zero was considered to be a number, one might have said something like 9 - 9 had no solution, though it'd still be clear that taking 9 parts away from 9 parts would leave no parts. However, using modern mathematics, "inventing" some solution for x = x + 1 would either result in the contradictory statement 0 = 1 or involve giving up the (very useful) field structure of the real numbers.

I wouldn't necessarily say the issue is trivial so much as deep enough in the foundations of mathematics that most mathematicians wouldn't worry about it in their day-to-day lives. What a mathematician would say in response to x = x + 1 is that given the standard definitions, there is no solution. If you forced the mathematician to fill in the blank with "The value of x is ______," I would imagine most would say "undefined" as mentioned previously, or perhaps some would say "not a real number."

### Zero = Nothing?

The OP does provide some context. The second equation is shown to result in 0 = 1. Thus, without redefining 0 or 1, the equation is invalid unless we're working in a structure like the zero ring, in which 0 and 1 coincide.

But since the equation is listed next to another one which mentions the element "4," I'm assuming we're not working within such a structure.

As for the OP's first question, I concur with Strange. The value of x is undefined.

For the OP's second question, I'd say that zero can be considered distinct from nothing. For example, the empty set {} is different from the set {0}. Of course, we could also say the first set contains zero elements, but then we're talking about cardinality of the set, rather than about the set itself. The empty set contains nothing, but doesn't contain zero.

So in the context of the equations in the OP, in the field of real numbers, the solution set for the first is {0}, whereas the solution set for the second is {}.

### What is INFINITY????

Definition 1 is the one I've seen most often.

Either definition is fine for the time being, though the second definition is only correct assuming the axiom of choice (which is, admittedly, pretty well-accepted these days). To explain further, the first part of definition 2 defines what is called a "Dedekind-infinite" set, and while every Dedekind-infinite set is infinite under Zermelo-Fraenkel set theory, we can prove the converse only if we invoke the AC (well, really just the axiom of countable choice, but eh). That is to say, without AC(ω), we cannot prove that every Dedekind-finite set is finite, so the second part of definition 2 fails.

### Aliens

We don't know the limits of technology just yet, and as zapatos says, a few thousand years is nothing compared to the age of the universe.

A sufficiently advanced civilization may have accomplished at least one of the following:

1. Overcome the difficulties involved in constructing something akin to an Alcubierre drive.

2. Construct an artificial intelligence or achieve whole brain emulation.

3. Perfect cryopreservation.

4. Avoid senescence and death.

Even without any of that:

1. Time dilation renders long (from our perspective) trips at relativistic speeds shorter from the perspective of the traveler.

2. An extraterrestrial's conception and experience of time may differ significantly from ours, such that a few thousand years of travel isn't unreasonable.

3. A spacefaring civilization could be nomadic, living on generation ships or something similar.

Of course, depending on their location and remote sensing capabilities, an alien civilization may not even notice our presence, much less decide to pay us a visit; and the probability that they'll randomly stumble upon us seems fairly low.

### Proof a negative times a negative equals a positive

f.e.

$-5*(-4)=+20$

$-5=\frac{+20}{-4}$

$-5=-5$ (true)

If you try the same with your assumption

"If we assume that (-1) (-1)=-1 and (-2)(-2)=-4"

it will fail.

Left side of equation, won't match right side.

This reasoning doesn't entirely work. If we assume -1 * -1 = -1, then we have

-5 * -4 = -20

-5 = -20/-4 = -20 * -1/4 = -5, as expected.

As mentioned earlier in the thread, what we do lose by letting -1 * -1 = -1 is the guarantee that certain rules of arithmetic (perhaps most notably, distributivity) extend to the negative numbers.

Edit: I actually should note that the entire notion of doing division involves an element having a multiplicative inverse, so my chain of operations doesn't necessarily work here either, since -4 * -1/4 would be -1 not 1. So we'd need to either give up the field structure of the real numbers or redefine multiplication a bit. Either way, this provides another example of why -1 * -1 = 1 is useful.

### Prove gamma function's trig identity?

Here's a resource related to the factorization of sine: http://www.ams.org/bookstore/pspdf/gsm-97-prev.pdf, and here's some more general stuff: http://www.math.umn.edu/~garrett/m/complex/hadamard_products.pdf.

To get a real understanding of all this, you'll probably want to study some analysis. I know I've enjoyed learning a bit about it in trying to answer your questions, but while I know enough to point in some potentially fruitful directions, I still don't know near enough to provide the clarification you seem to want.

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