# Keen

Members

29

6 Neutral

• Rank
Quark

## Profile Information

• Favorite Area of Science
Mathematics
1. ## How is time a dimension?

My understanding of relativity is very limited, so I'd appreciate if someone could help me understand this concept. When it comes to an object, I can move it in space by applying forces change direction etc... I can even make an object stop moving altogether in a certain frame of reference, but when it comes to time, it seems to me that there is little control over how an object moves in time. It always goes forward, I can't make the object stop etc... so how is it helpful to think of time as a dimension when it behaves very separately from the others?
2. ## Way ahead for Trig Integral

If you do not want to use Wolfram, you're going to have a difficult time calculating this integral. The general method for calculating these kinds of integrals is using following formulas: $sin(x)=\frac{2tan(\frac{x}{2})}{1+tan(\frac{x}{2})^2}$ and $cos(x)=\frac{1-tan(\frac{x}{2})^2}{1+tan(\frac{x}{2})^2}$ You then change the integration variable by putting $u=tan(\frac{x}{2})$ and you should obtain a rational function in u. You then decompose it into partial fractions and integrate these. However considering how many different constants there are, it is likely to take enormous time and I definitely am not motivated to do this kind of calculations.
3. ## Discriminant with inequality proof

This is typically a statement that is best proven by contraposition. Assume that the discriminant is positive and then prove that your polynomial cannot be non negative in all points. When you have a positive discriminant, your polynomial has exactly two roots and depending on the sign of a, it is either positive between these roots or negative. If it is negative, you're done. If not then it must be positive between the largest of the two roots and infinity and between minus infinity and the smallest of your two roots. I guess you could also prove it directly, but then you'd probably have to waste time with who knows how many different cases. Hope it is clear enough.
4. ## integration

The best hint I can give you is that for any interval I and any function y positive on I: dy/dx=y (d ln(y)/dx) with ln being the natural logarithm. With that, you should be able to solve it.
5. ## Pushing, pulling and "dualing"

Look. You could call me an advocate of mathematics for the sake of mathematics and I am not criticizing the fact that you are exposing theories that require a higher level of abstraction. What I am criticizing is your approach at exposing them. For me to consider some theory interesting, it must either have some sort of problems or solve other problems from other fields. Also usually I find a much better approach when explaining abstract mathematics: to give a lots of examples to make your audience grasp what kind of objects you are dealing with. Also what does your abstraction generalize? Does this generalization help to tackle some problems? And so on... I once took courses in Arakelov geometry, which were unfortunately pretty useless to me, because our teacher made something similar to what you are doing right now. He started with some very abstract notions like Grothendieck topology or some very general functorial inductive limits (not the usual ones with a directed set), without ever motivating these notions, so most students got lost in these abstractions pretty quickly. It's a pity that you are over dramatizing people's criticism and lack of enthusiasm, because I am sure there are interesting things to be learned from your posts. When it comes to category theory I usually prefer to use the Godel-Neumann class theory. It's most likely just a cosmetic change to talk about the class of all sets instead of Grothendieck universe, but in theory it should be larger since it encompasses all the sets, while Grothendieck universe only encompasses the sets we "care" about. I doubt it changes much in practice, but it's a thing I wanted to point out.
6. ## Pushing, pulling and "dualing"

We might not have the same notion of fun.... I think it would be much more interesting if you showed some problems that naturally arise from what you just showed or at least some reason why this deserves any attention. Perhaps you could show some concrete vector spaces and give an example of a specific pull-back or push-forward? Otherwise it just seems (at least to me) like some pointless abstract symbol manipulation.

8. ## Credit cards and digital signature

I suppose yeah. As soon as the client does not see much difference or the security measures become too technical, credit card companies will probably not want to invest in it. Being an amateur enthusiast in cryptography I just find it disappointing to see a system where you send the same secret data to verify identity being used while there are in my opinion much more secured ways. There are some banks, which send a one time password by sms, which is already quite a good security measure, but I'd still like to see a system with something like zero knowledge proofs or digital signatures implemented simply because I don't like much the idea of giving my card number to strangers.
9. ## Credit cards and digital signature

I'm not sure whether to put this in the computer science section or applied mathematics, but I've always considered cryptography to be applied mathematics, so I will post it here. I don't unfortunately know all the details behind how online transactions work, but as far as I know, you send to the merchant your credit card number and cryptogram and he uses those informations to validate the transaction. This seems to me a bit insecure, because you have to trust the merchant as you are giving him all the necessary information that can be used to pay anywhere and if someone for example steals it from him, he could reuse it somewhere else. (Yes I am aware that the communication is encrypted, so stealing that number isn't that easy). An alternative, that to me sounds much more secure is to use the credit card number as a private key in some digital signature algorithm like for example DSA. That way, the merchant sends you all the necessary information for the transaction like for instance some identification, price to pay, date etc.. you digitally sign it with your credit card and then send it back to the merchant. That information can be then validated by the payment server and cannot be reused by anyone else, since it is a digital signature of only one specific transaction. If needed, this could most likely be as well adapted for monthly payments. You would simply send a monthly payment order signed by your digital signature to a company like netflix instead of your credit card. Maybe I'm not getting something, but that to me seems much more secure, than sending a simple unchanging information over the internet.
10. ## Does this hold true for all Prime products? / RSA isn' the encryption...

Or you could encrypt the private key by some symetric encryption algorithm like AES or blowfish and decrypt it in your RAM only when needed. That's how for example PGP works if I'm not mistaken.
11. ## recursive numbering and symmetric numbers

Very interesting idea, but unless I did not get something, I believe your first hypothesis to be false. You claim that numbers which differ by one have no more than two brackets of difference between them in their bracket notation. I tried among the Fermat's primes as they grow rather quickly and it's easy to decompose the preceeding number. I have found $257=2^8+1$. 257 is 65th prime, so according to your notiation it should have 65 opening brackets and 65 closing brackets. The preceeding number $256=2^8$ has eight twos in it's decomposition therefore if I get it correctly it should have 8 opening and 8 closing brackets, which is quite a difference with the next number. I'd personally speculate that there is no bound for the difference between brackets of consecutive numbers simply because when you multiply small prime numbers among them the result tends to grow rather quickly and if the consecutive number is a prime then it would be a rather large prime with lots of brackets, while the preceding number won't have that many brackets, since it's composed of a relatively small number of small primes.
12. ## Continuous set and continuum hypothesis

That would probably be for the best. I don't want you to discourage from mathematics, but if you really wanted to write a serious paper on such a very difficult topic as continuum hypothesis, it would take years of study in mathematics and then even more hard work in mathematical logic. Either way as far as I know, continuum hypothesis is independent of ZFC axioms, which is a standard framework for modern mathematics, so you can't neither prove nor disprove it: you can only make a theory based on it or its negation. I am sure there are lots of fascinating results in this kind of non standard mathematics, but I am unfortunately not knowledgeable enough in mathematical logic to discuss this topic further.
13. ## Cardinality of the set of binary-expressed real numbers

Ok, I thought you were considering this type of numbers. I just wasn't sure. It actually does make perfect sense. You just consider the infinite sequence (un) of 0 and 1 and to this sequence you can associate a unique number which is the limit of the infinite series $\Sigma$ un 2-n. On the other hand your limit does not make much sense. You claim, that by making n going to the cardinal of aleph0, you obtain the result, that the cardinal of numbers with finitely many digits is aleph1. However you still do not explain in what sense you take your limit. It surely isn't the classical limit of sequences, since the limit of 2n in the classical sense is simply infinity, which does not say then anything on the cardinality of your set $B_F$, besides that it is infinite, but that's not really what you are trying to prove. Also I would like to add, that the notation of the powerset $2^{\mathbb{N}}$ is a mere notation and isn't anything particularly special, which makes your use of limit even weirder and if I am to be honest, I do not think there is a way to write your limit formally in a way that would be compatible with modern mathematics as it is a very well known result, that a set can't have the same cardinal as its powerset. The proof of this statement is actually not that difficult and is reminiscent of Bertrand Russel's paradox, which I recommend you to check out. If there was a set A with the same cardinal as its powerset, there would be a one to one correspondence between A and $2^A$, let's call it f. You can think of one to one correspondence as a label. To each member x of A, you associate a unique subset of A denoted f(x). One to one simply means, that each subset of A has a unique label. Now we consider a very particular subset of A, which has all the elements x, such that x is not a member of f(x). We call this subset B. Since we know that f is one to one, it means that there is y in A such that B=f(y) and now the question is whether y is in B or not. First let's suppose it is. Then by definition of B y is not in f(y). but f(y) is precisely B. If on the other hand it isn't in B, that means that y is in f(y) by definition of B, but f(y) is B, so y is in B, which is a contradiction. No matter how you put it you obtain something contradictory, which means that the set B does not exist and therefore neither the one to one correspondence f does.
14. ## Cardinality of the set of binary-expressed real numbers

Well honestly I am kind of lost already in your first paragraph. When it mentions "We let n go to the cardinal number of the set of natural numbers", what does that thing even mean? From what I understand, what is called here a binary fractional number, is pretty much a binary number with finite decimals, which actually is simply a rational number. Claiming, that this set has the cardinal of the powerset of natural numbers is a very bold assertion, since it challenges already well established results, so this magical use of limit "We let n go to the cardinal number of the set of natural numbers." had better be very well explained. The problem is that with limits you can make pretty much anything if you don't use them carefully and here, I do not think there is a sufficient justification behind that use of limit. If on the other hand the set of binary fractional numbers is the set of all numbers in [0,1[ written in binary, then it seems to me there is a misrepresentation, because the process that is used to list those numbers only deals with numbers with finite decimals. So yeah it's all quite messy and needs rewriting and I am almost 100% sure, that if those arguments were written correctly a flaw would quickly appear, because one does not simply contradict Cantor's diagonal argument.
15. ## Are there any good physics books with a more mathematical approach?

Hi. I am not sure whether to put this topic in physics or mathematics, since it's kind of both. I used to take physics courses back when I was an undergraduate student and unfortunately I didn't like them much. Mainly because of how the mathematical models were treated 'poorly'. By that I mean that we lacked rigorous definitions and I wasn't even sure of for instance how regular the functions that we employed were. I once stumbled upon a book whose name I have unfortunately forgotten, but I remember it was about special relativity and it was really written for people with my mindset. To give you an example, it defined a material point as a couple (gamma, m) with gamma an infinitely differentiable curve in a Minkowsky space and m a positive real number. I know that physics is supposed to represent real world and I don't mind that. I just think that as soon as physicists use mathematical representations, they should define their objects well... mathematically. I wanted to ask if you know of other books which take this kind of formal approach to physics? I am mainly interested in classical and relativistic mechanics, electromagnetism and thermodynamics. Again I don't mind references to experiments and physical explanations: it's physics after all, but I would just like that all the formal mathematical part is treated "correctly".
×