# A $1,000,000 Question by Canute Tags: None HW Helper Sci Advisor P: 1,996  Quote by Canute Yes, I think it was. This leaves my question. If the probability of N+/-1 being a twin prime falls (all the way) to zero as P increases then there are not infinitely many twin primes of the form N+/-1. If it does not then there are. Is this reasoning flawed? Or, if the probability does not fall (all the way) to zero then does it just mean there may be infinitely many? If the density of the sequence goes to zero, then there may or may not be infinitely many elements. The squares example (or the primes themselves) have density going to zero, yet there are infinitely many of them. Any finite set will have it's density going to zero. If you have at least one element, then the density never actually equals zero. For example the probability of selecting an even prime from {1, 2, ..., x} is 1/x for all x. No matter how big x is, it's never zero. If it does not go to zero, then there must be infinitely many of them. P: 1,499  Quote by shmoe If the density of the sequence goes to zero, then there may or may not be infinitely many elements. Hmm. I thought perhaps that it was possible to show that the probability of N+/-1 being (both) prime never becomes zero, and that this would mean there are infinitely many twin primes, but it looks like this approach will not work. I think I need a bit of time to digest what I've learnt here. It has all been very helpful, and thanks for taking the time to explain so much. Can we leave it here for the moment, as far as twin primes go? I'll come back with some more questions when I've given more thought to what you've said so far. Would you be able to clarify for me the relationship between the non-trivial zeros of RH's function and the distribution of primes? Or do I need to know more mathematics before you can do that? Also, am I right to suppose that RH must be proved by reference only to the zeta function itself, or would the proof have to depend in some way on the actual behaviour of the primes? HW Helper Sci Advisor P: 1,996  Quote by Canute Hmm. I thought perhaps that it was possible to show that the probability of N+/-1 being (both) prime never becomes zero, and that this would mean there are infinitely many twin primes, but it looks like this approach will not work. It looks to me like the density will be going to zero, but the heuristic argument could be off (though I would doubt it to be that off). You might want to search around to see how far people have gone with an exhaustive effort on primorial primes (the N+/-1) and factorial primes (these a n!+/-1). The P<12000 was a few years old I think. More data would be good to compare the heuristic against.  Quote by Canute Would you be able to clarify for me the relationship between the non-trivial zeros of RH's function and the distribution of primes? Or do I need to know more mathematics before you can do that? Also, am I right to suppose that RH must be proved by reference only to the zeta function itself, or would the proof have to depend in some way on the actual behaviour of the primes? See http://www.physicsforums.com/showthread.php?t=88468 for now, the last post explains the "explicit formula" (<-words to google) a little bit. That gives it in the simplest version for you, in terms of the function that counts primes as well as prime powers with a logarithmic weight rather than the usual pi(x). The counting function is more complicated, but the right hand side is much simpler that what you would have seen in Derbyshire's book. Sadly the link I give at the bottom doesn't seem to work right now, I'll hopefully be able to find a new one. http://www.physicsforums.com/showthread.php?t=73459 explains a little bit about the equivalence of the error term in the prime number theorem vs locations of zeros. Also search around this forum, this has been talked about more than just those posts. I'm taking the lazy route of links now, but I'll be able to expand more later. RH could theoretically be proved with no reference to the zeta function itself. The theory of the zeta function does rely on the euler product, partly a consequence of unique factorization, so most of it can't be considered independant of the primes. However, this is really the only location it turns up, and any zeta results you can prove by first proving something about the prime distribution are much weaker than you get with the complex theory itself.  HW Helper Sci Advisor P: 9,371 this reminds me of that show "who wants to be a billionaire?", put on by the same people who made candid camera i think.  P: 81 Cool stuff here. Regarding the probability going to zero, I have two examples that illustrate opposite extremes. The first is the set {1, 2, 3}. If you look at the probability that a natural number below x is in that set, the probability will get as close to zero as you could desire, but it would still never actually reach zero, despite being a finite set. The other example is the probability that a real number will be a natural number {1, 2, 3, ...}. Here the probability is exactly zero that a given number will be a natural number, regardless of the interval. However, there are infinitely many natural numbers. This is due to the fact that real numbers form an uncountable infinite set, whereas natural numbers form a countable infinite set. These examples are intentionally ludicrous - to demonstrate that you cannot say anything about the size of a set based on probabilities. What caught my eye more than anything else, though, was the mention of fourier with the primes. I once looked at the sum over primes of (1/p)(sin p). It probably would not be very pleasant to listen to, though, with all the discontinuities. P: 1,499  Quote by mathwonk this reminds me of that show "who wants to be a billionaire?", put on by the same people who made candid camera i think. Yeah, I thought mention of$s would liven things up. I wish I hadn't done it now.
 P: 1,499 Thanks for the links. I should have searched the threads before posting anything. I came across this. "The first instance of this to be observed involved the Selberg trace formula (discovered in the 1950's) which concerns the geodesic flow on a Riemann surface, relating its periodic orbits and its energy levels, i.e. eigenvalues of the Laplace-Beltrami operator. Here the orbits correspond to the primes and the energy levels to the Riemann zeta zeros. The latter correspondence lends credence to the spectral interpretation of the Riemann zeta function, and the overall situation suggests the existence of some kind of mysterious dynamical system underlying (or "lurking behind" as N. Snaith put it in her Ph.D. thesis) the distribution of prime numbers." This is the sort of comment that confuses me. What is mysterious about the dynamical system lurking behind the distribution of primes? It doesn't seem mysterious to me, so what am I overlooking? "The wider phenomenon of correspondence between the explicit formulae of number theory (of which the Riemann-Weil formula is just one, important, special case) and dynamical trace formulae points to some fundamental issue of duality which is currently a great mystery, and may turn out to be hugely significant in our understanding of both mathematical and physical reality." What is this issue of duality? Or is it inexplicable at my level of mathematics?
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P: 1,996
 Quote by Canute This is the sort of comment that confuses me. What is mysterious about the dynamical system lurking behind the distribution of primes? It doesn't seem mysterious to me, so what am I overlooking?
I don't think you have the same definition of a dynamical system. I don't know this kind of stuff well enough to offer anything resembling a plain english explanation, so this will be very vague.There is some hope that the zeta function will be attatched to some kind of dynamical system with some nice properties. If we knew what this dynamical system was (we don't know, though there are many clues to it's behavior), the hope is a proof of the riemann hypothesis will follow. I said it would be vague.

 Quote by Canute What is this issue of duality? Or is it inexplicable at my level of mathematics?
The explicit formula for the prime counting function is an example of this kind of duality. We have two objects, the primes and the zeros of zeta, and we have a connection between sums over these objects. This allows you to translate results about one of these objects to the other. For example you can go back and forth between smaller error term for the prime number theorem and larger zero free regions for zeta. Historically we 've always used an improved zero free region to prove a sharper error term, though the reverse is theoretically possible.
 P: 1,499 But the quote says a dynamical system underlies the distribution of prime numbers, not the zeta function.
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P: 1,996
 Quote by Canute But the quote says a dynamical system underlies the distribution of prime numbers, not the zeta function.
See the 'duality' bit again, these things can't be seperated. If something is connected to zetas zeros, then it's connected to the primes. Here the connection between the primes and this hypothetical dynamical system lies through the zeta function- it will be more 'natural' to connect this system (which we don't even know exists) with the zeta function than directly to the primes.
 P: 1,499 That may be so, but it doesn't seem to alter the original statement except to say that this supposed dynamical system underlies both the distribution of primes and the zeta zeros. It still says that such a system might underly the primes, and presumably would have done so long before the zeta function was invented. I still find the statement odd. Do mathematicians mean the same by 'dynamical system' as physicists?
 HW Helper Sci Advisor P: 1,996 Yes, same as physicists. Maybe I should ask you, if you don't find it mysterious, please explain it to us!
 P: 1,499 Well, I'm sure this will be a misunderstanding on my part and this system is mysterious after all, but this is what I meant. The distribution of primes is caused by the distribution of multiples of primes. (I.e if a number n at 6n+/-1 is ~prime then it is because it is a multiple of a prime at or below sqrtn). Only two in every six multiples of a prime p have any effect on the distribution of primes above 6p. These multiples can be predicted. (The quantity of relevant multiples of a prime p in a range R of numbers is R/3p. E.g. the quantity of multiples of 101 occuring at 6n+/-1 between 606 and 1212 is 2). In calculating pi(x) the complication is that these multiples cannot simply be summed. A correction term is required (because many non-primes at 6n+/-1 are multiples of more than two primes). This correction term is complex, but I can't see what's mysterious about the mechanism.
 HW Helper Sci Advisor P: 1,996 That is not a 'dynamical system'. What you've just described is how a basic sieve works. The correction term can be worked out (I mentioned 'inclusion-exclusion' before) and allow you to find pi(x) given say a list of primes less than sqrt(x), this has been known all the back to Legendre, sieves themselves back to Eratosthenes. This is definitely not what Snaith is refering to as mysterious. I do have some idea how the zeros are supposed to be related to some unkown Hermitian operator (i.e. the ramndom matrix theory stuff), but I don't think I have a chance at properly explaining how this operator will be connected to a dynamical system. There's the paper by Berry and Keating "The Riemann-Zeros and Eigenvalue Asymptotics", SIAM Review, vol 41, no. 2, pp 236-266 that goes into some detail on what this dynamical system will probably look like, I've been meaning to give it a thorough read but haven't got around to it yet.
 P: 1,499 As usual I'm not understanding something here. The original comment I quoted speaks of the primes, not the zeta zeros. I realise that what I described does not constitute a dynamic system, nevertheless it is the rule or mechanism that determines the position of primes. If this mechanism is not the system spoken of, then how can there be another one? Any dynamic system would have to produce the same outputs. Does the quoted remark mean that this mechanism can be modelled as a dynamic system?
 HW Helper Sci Advisor P: 1,996 Sure, it's a rule that determines where the primes are, but remember one of the goals of number theorists is to improve the error term in the prime number theorem. Sieves and other elementary methods (essentially meaning no complex analysis) have produced some results, but nothing like we can do with the zeta function as far as the error term goes. Yes, zeros and primes are the same thing in a sense, so if you find out what's up with one of them you know about the other. In this sense this dynamical system would tell you about the primes, and can be thought of the thing controlling them (though you could probably think of the primes as controlling the dynamical system), so you could think of it as 'the same thing' as what you've described in a way. However, if you ran into this mythical dynamical system in a back alley it would probably be impossible to draw a connection between it and primes without the zeta function in hand to translate back and forth. This is the sense that I mean the dynamical system is more naturally attatched to zeta and not the primes.
 P: 1,499 Hmm. I still can't understand why this system is said to be mysterious, but never mind. I think I need to stop here and go away to think some things through, now you've explained some of the mathematics to me. Thanks for all your help and patience. I'll put the rest of my thoughts in better order and may be back to ask you some more questions. I can at least now see the direction I need to head in. Many thanks Canute
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