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I've read so much about it but none of it makes a lot of sense. Also, what's so unsolvable about it?

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The Riemann Hypothesis isn't something that can be easily explained in a short answer unless you have some background in complex analysis, because otherwise its statement, as you just said, doesn't make a lot of sense. And since you said to have read a lot about it without making much progress, I'm not really sure if you'll get something better from here. But, let's be optimistic and see what happens. –  Adrián Barquero Oct 27 '10 at 1:47
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Here are a couple of links to non-technical articles: "The Music of the Primes" by Marcus du Sautoy, "The Spectrum of Riemannium" by Brian Hayes. –  Rahul Oct 27 '10 at 2:08
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Also, this previous question is pretty similar. –  Rahul Oct 27 '10 at 2:10
    
Slightly watered down, but here is something you might want to look at. –  J. M. Oct 27 '10 at 2:25
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"What's so unsolvable about it?" is a funny question to ask. The only appropriate answer I can think of is that it happens not to have been solved yet, despite $150$ years of efforts on behalf of many people. Other than that, it might not be unsolvable at all! –  Pete L. Clark Oct 27 '10 at 7:19
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7 Answers 7

up vote 56 down vote accepted

The prime number theorem states that the number of primes less than or equal to $x$ is approximately equal to $\int_2^x \dfrac{dt}{\log t}.$ The Riemann hypothesis gives a precise answer to how good this approximation is; namely, it states that the difference between the exact number of primes below $x$, and the given integral, is (essentially) $\sqrt{x} \log x$.

(Here "essentially" means that one should actually take the absolute value of the difference, and also that one might have to multiply $\sqrt{x} \log x$ by some positive constant. Also, I should note that the Riemann hypothesis is more usually stated in terms of the location of the zeroes of the Riemann zeta function; the previous paragraph is giving an equivalent form, which may be easier to understand, and also may help to explain the interest of the statement. See the wikipedia entry for the formulation in terms of counting primes, as well as various other formlations.)

The difficulty of the problem is (it seems to me) as follows: there is no approach currently known to understanding the distribution of prime numbers well enough to establish the desired approximation, other than by studying the Riemann zeta function and its zeroes. (The information about the primes comes from information about the zeta function via a kind of Fourier transform.) On the other hand, the zeta function is not easy to understand; there is no straightforward formula for it that allows one to study its zeroes, and because of this any such study ends up being somewhat indirect. So far, among the various possible such indirect approaches, no-one has found one that is powerful enough to control all the zeroes.

A very naive comment, that nevertheless might give some flavour of the problem, is that there are an infinite number of zeroes that one must contend with, so there is no obvious finite computation that one can make to solve the problem; ingenuity of some kind is necessarily required.

Finally, one can remark that the Riemann hypothesis, when phrased in terms of the location of the zeroes, is very simple (to state!) and very beautiful: it says that all the non-trivial zeros have real part $1/2$. This suggests that perhaps there is some secret symmetry underlying the Riemann zeta function that would "explain" the Riemann hypothesis. Mathematicians have had, and continue to have, various ideas about what this secret symmetery might be (in this they are inspired by an analogy with what is called "the function field case" and the deep and beautiful theory of the Weil conjectures), but so far they haven't managed to establish any underlying phenonemon which implies the Riemann hypothesis.

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@Jonas Meyer: Dear Jonas, Thanks for fixing my unfortunate typo! –  Matt E Oct 27 '10 at 4:39
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A direct translation of RH (Riemann Hypothesis) would be very baffling in layman's terms. But, there are many problems that are equivalent to RH and hence, defining them would be actually indirectly stating RH. Some of the equivalent forms of RH are much easier to understand than RH itself. I give what I think is the most easy equivalent form that I have encountered:

The Riemann hypothesis is equivalent to the statement that an integer has an equal probability of having an odd number or an even number of distinct prime factors. (Borwein page. 46)

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Is there an explanation for this equivalence online somewhere? Since the prime factorization of some integer is not random at all, I have a hard time getting what probability has to do with it. (Unless you choose the integer at random, at which point I have to ask: "By what distribution?"). –  Jens Oct 27 '10 at 6:32
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I suggest you take a look at the book "The Riemann Hypothesis: A Resource for the Afficionado and Virtuoso Alike" by Borwein, Choi, Rooney and Weirathmueller. In theorem 1.2 for the relation of Riemann's Hypothesis with Liouville Lambda function, it is stated that RH is equivalent to $$\lim_{n\to\infty} \frac{\lambda(1) + \lambda(2) + ... + \lambda(n)}{n^{\frac{1}{2} + \epsilon}} = 0$$ for every fixed $\epsilon > 0$. I think you can search up topics of Liouville's lambda function and riemann hypothesis on the net. –  Roupam Ghosh Oct 27 '10 at 6:51
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Here's another elementary statement equivalent to the RH: en.wikipedia.org/wiki/Jeffrey_Lagarias –  Hans Lundmark Oct 27 '10 at 9:45
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In very layman's terms it states that there is some order in the distribution of the primes (which seem to occur totally chaotic at first sight). Or to say it like Shakespeare: "Though this be madness, yet there is method in 't."

If you want to know more there is a new trilogy about that topic where the first volume has just arrived: http://www.secretsofcreation.com/volume1.html

It is a marvelous and easy to understand book from a number theorist who knows his stuff!

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You have to...have to....read this friendly introduction. (Making it CW since it's just a link.)

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reading about the context of the RH makes me understand it A LOT more. even though a lot of people explain it differently and some even makes sense, actually learning about the context and why it is important really really helped me, thanks for posting this! –  Letseatlunch May 28 '13 at 23:31
    
I use this friendly guide because it is funny, approachable, but correct. I especially enjoy the "I'm Jealous" chapter. –  John Washburn Jan 8 at 17:00
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Here is a very simply description of the Riemann Hypothesis that requires nothing more than a 3rd grade education to understand:

http://www.jstor.org/pss/2323497

There is also a beautiful proof linking the Farey sequence of fractions to the Riemann hypothesis by Jerome Franel. It's only three pages long and should be able to be understood by any undergraduate mathematics major.

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Let $H_n$ be the nth harmonic number, i.e. $ H_n = 1 + \frac12 + \frac13 + \dots + \frac1n.$ Then, the Riemann hypothesis is true if and only if

$$ \sum_{d | n}{d} \le H_n + \exp(H_n)\log(H_n)$$

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It is straight forward. We have a Zeta function, 'analytically continued' that says

Z(s)[1-2/2*s] = 1 - 1/2*s + 1/3*s - 1/4*s + ..... to infinity. Here symbol '**' means 'raised to the power' and s is a complex variable. Thus s= real part(sigma) + i times imaginary part(omega). The above series converges in the region of our interest which is 0 < sigma < 1.

To find the zeroes of Z(s) we need to first substitute Z(s) = 0 and solve for s and thus sigmas and omegas.

That is

Solve 0= 1 - 1/2*s + 1/3*s -1/4**s + ..... to infinity, for sigmas and omegas. Riemann hypothesized that the zeros will have their sigmas equal to 1/2 while the omegas are distinct. To this date, after 150 years, no one has any clue why sigma takes a single value 0f 1/2 in the critical strip 0 < sigma < 1. Apart from the consequences I hope I explained it well. Wikipedia on Riemann Hypothesis is a good source for reading up.

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Welcome to MSE! It really helps to format posts using MathJax (see FAQ). Regards –  Amzoti Mar 27 '13 at 19:59
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