# A very slowly diverging series

It is known and quite easy to prove that $$S_{\mathbb N}(x) = \sum_{n\in\mathbb N, n\leq x} \frac 1 n$$ grows as $$\ln x$$. Even more, $$\lim_{x\rightarrow\infty} S_{\mathbb N}(x)-\ln x = \gamma$$, the Euler-Mascheroni constant.

It was also proved by Mertens that $$S_{\mathbb P}(x)$$, the sum of reciprocals of the primes not exceeding $$x$$, grows as $$\ln\ln x$$, and actually, $$\lim_{x\rightarrow\infty} S_{\mathbb P}(x)-\ln\ln x = M$$, the Meissel-Mertens constant.

My questions is, is there an example of a set $$A\subset \mathbb N$$, such that $$\lim_{x\rightarrow\infty} S_A(x)-\ln\ln\ln x=C$$ for some real $$C$$. It is quite obvious that such a set exists, however I am looking for some "natural" examples (maybe some that were not specifically constructed as an answer to this question, but rather appeared during some research).

• Are there are "natural" sets with density $1/\log(X)\log\log(X)$? – MyNinthAccount Oct 30 '19 at 19:14
• Maybe: the primes $q$ such that $q-1$ has no odd prime divisor up to $C\log(q)$ (unsure what the constant $C$ should be). – MyNinthAccount Oct 30 '19 at 19:24
• Maybe: the $p$th primes where $p$ itself is prime? E.g. 3, 5, 11, 17, 31, ... – Aeryk Oct 31 '19 at 2:40
• @Aeryk as it turns out, the $n$th term of that series is asymptotically $p_n \log p_n \sim n \log^2 n$, and so the sum of its reciprocals converges. – Greg Martin Oct 31 '19 at 7:33
• This does not work, but perhaps something along the lines might: I think that the integer series $np_{p_n}-p_n^2-(n-1)p_{p_{n-1}}+p_{n-1}^2$ may on average grow like $2p_n\log\log p_n$, but its individual values fly around and are often negative. – Yaakov Baruch Oct 31 '19 at 13:35

## 1 Answer

A nice example was found by Erdos; "On a problem of G. Golomb". Let $$p_1 = 3$$, and for $$i > 1$$, let $$p_i$$ be the least odd prime exceeding $$p_{i-1}$$ which is not congruent to $$1$$ mod $$p_{j}$$ for any $$j < i$$. That this sequence of primes has the property you seek follows from eq. (37) there (take logarithms).