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I don't know whether this series converges: $$(\frac{1}{4} - \frac{1}{5}) + (\frac{1}{6} - \frac{1}{7}) + (\frac{1}{8} + \frac{1}{9} + \frac{1}{10} - \frac{3}{11}) + (\frac{1}{12} - \frac{1}{13}) + (\frac{1}{14} + \frac{1}{15} + \frac{1}{16} - \frac{3}{17}) + \dots$$ Can you explain it to me?

Definition of terms

If $n$ is composite you have $1/n$ and if $n$ is prime you have $−(n−m−1)/n$ where $m$ is the previous prime.

Details

Actually I got the series from the equalities: $$\log(n)=\sum_{i=1}^m \frac{p_i-p_{i-1}} {p_i}+\epsilon_n,$$ $p_i$ denotes prime number from $1$ to $n$, $p_0$ is $1$ and $p_m$ is $n$ and

$$\sum_{i=1}^n \frac{1} {i}=\log\left(n\right)+\gamma+\delta_n.$$

Obviously $\epsilon_n$ becomes larger and larger when $n$ increases, but it increases very slowly. The value is $0.706540\ldots$, gotten by computer. What I want to know is whether $\epsilon_n$ has a upper boundary.

  • What do you mean by 'finite'? Does it converges? – brick Nov 28 '14 at 15:51
  • Yeah, I mean that. – Xuefeng Meng Nov 28 '14 at 15:52
  • Welcome to the site. Could you please try to include a general desciption of the series, verbal or formal. One can sort of figure out how it is form, but it is a bit of guessing. You can [edit] the question via following the link, or clicking the button below the post. – quid Nov 28 '14 at 15:53
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    And how do $3/11$ and $3/17$ appear, what is the logic? Shouldn't they be $1/11$ and $1/17$? – brick Nov 28 '14 at 15:54
  • 3 means the gap between 7 and 11(actually 4) minus 1, and other items is determined like this way. – Xuefeng Meng Nov 28 '14 at 16:00
  • And do the negative signs correspond to prime numbers in the denominator, and will you eventually come to $-\cfrac 5{29}$? – Mark Bennet Nov 28 '14 at 16:03
  • As noted by other users, the general term of the series is (still) undefined hence, strictly speaking, there is no question (yet). – Did Nov 28 '14 at 16:05
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    Do you mean that if $n$ is composite you have $1/n$ and if $n$ is prime you have $-(n-m-1)/n$ where $m$ is the previous prime? – Robert Israel Nov 28 '14 at 16:05
  • Still can't get it. Can you write it formally using letters, not specific numbers? – brick Nov 28 '14 at 16:05
  • Yeah,Robert Israel has got what I mean. – Xuefeng Meng Nov 28 '14 at 16:09
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    I'd say a really good exercise here would be to ask whether there are infinite negative terms with numerator $;1;$ ...:) – Timbuc Nov 28 '14 at 16:25
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    Can you explain where you found this series, why it is interesting to you and give some general information about the context. – Mark Bennet Nov 28 '14 at 16:55
  • Got something from the answer? – Did Jan 04 '15 at 15:42
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    I think I can prove that $p_{k+1}$-$p_k$ < $p_k^{\frac{1} {2}}$ by using the series.And this lead to Oppermann's Conjecture. – Xuefeng Meng Jan 06 '15 at 02:47
  • XuefengMeng: Not my question. Got something from the answer? – Did Jan 06 '15 at 08:11
  • You mean your answer? – Xuefeng Meng Jan 06 '15 at 09:29
  • I think the series is interesting and we can generalise it.What do you think of it? – Xuefeng Meng Jan 07 '15 at 15:17
  • What I think of it is mainly that you do not play by the rules of the site, which include the following: 1. Ask full questions from the start (as opposed to, forcing readers to make you complete them). 2. When a full answer is posted, after a while accept it (as opposed to, invoking unspecified follow-ups to the question actually asked). 3. When you post a comment to some user, use @ in the comment. – Did Jan 10 '15 at 18:59

1 Answers1

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(Now a full solution, thanks to @GregMartin, see the end of the answer.)

Let $(p_n)_{n\geqslant1}$ denote the sequence of prime numbers, hence $p_1=2$, $p_2=3$, $p_3=5$, and so on, and let $g_n=p_{n+1}-p_n$ the $n$th prime number gap. From the exchanges in comments, it seems that the question asks about the convergence of the series $$A=\sum_{i\geqslant4}\frac{a(i)}i,\quad a(p_n)=1-g_{n-1}\ (n\geqslant3),\quad a(i)=1\ (i\ \text{not prime}).$$ The fact that $g_n\ll p_n$ implies that the oscillations of the partial sums of $A$ converge to zero hence the convergence of $A$ is equivalent to the convergence of the series with positive terms $$\sum\limits_{n\geqslant2}s(p_n,g_n),$$ where the functional $s$ is defined on every pair $(p,g)$ of positive integers by $$s(p,g)=\sum_{i=1}^{g}\left(\frac1{p+i}-\frac1{p+g}\right).$$ Note that, for every positive $p$ and $g$, $$s(p,g)\leqslant\sum_{i=1}^{g}\left(\frac1{p}-\frac1{p+g}\right)=\frac{g^2}{p(p+g)}\leqslant\frac{g^2}{p^2}.$$ Likewise, $$s(p,g)\geqslant\sum_{i=1}^{g/2}\frac1{p+g/2}-\frac1{p+g}=\frac{g^2}{4(p+g/2)(p+g)}\geqslant\frac{g^2}{4(p+g)^2},$$ hence the convergence of $A$ is equivalent to the convergence of $S_2$, where, for every $\alpha$, $$S_\alpha=\sum_n\left(\frac{g_n}{p_n}\right)^\alpha.$$ Thanks to @GregMartin, one knows that the series $S_\alpha$ converges for every $\alpha\gt1$, in particular $S_2$ converges. This proves that the series $A$ converges.

Did
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  • Question related to this unfinished proof. – Did Nov 28 '14 at 18:44
  • Thanks for your answer. Your analysis is absolutely the same as what I dealt with the series when I had got it. – Xuefeng Meng Nov 29 '14 at 01:11
  • Actually I got the series from the equalities:log(n)=\sum_{i=1}^m \frac{p_i-p_(i-1) {p_i}+epsilon_n, (p_i denotes prime number from 1 to n, p_0 is 1 and p_n is n) and \sum_{i=1}^n \frac{1} {i}=log(n)+gamma+delta_n.\Obviously,epsilon_n becomes larger and larger when n increases, but it increases very slowly. The value is 0.706540... got by computer. What I want to know is whether epsilon_n has a upper boundary. – Xuefeng Meng Nov 29 '14 at 01:34
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    Yeah, why did you omit the context from your question (now included as "Details") until after I posted my answer? – Did Nov 29 '14 at 07:33
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    I've posted an answer to the related question that establishes the convergence of $\sum g_n^2/p_n^2$ and hence of $A$. – Greg Martin Nov 29 '14 at 09:12
  • @GregMartin Thanks for your answer, it is nice. – Did Nov 29 '14 at 13:09
  • @Did-I am sorry about this. I am a new comer here. Next time I would post the context when I ask a question. – Xuefeng Meng Nov 29 '14 at 14:29
  • @Greg Martin But I think that there is an error in the first step of your analysis of the related question which Did posted. Your answer couldn't convince me. – Xuefeng Meng Nov 29 '14 at 15:21
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    XuefengMeng: Unless you explain why @GregMartin's answer "couldn't convince" you, this seems to be more your problem than GregMartin's problem. – Did Nov 29 '14 at 18:01