# [seqfan] Re: Help with a calculation for A001097 (twin primes)

Allan Wechsler acwacw at gmail.com
Wed Nov 19 17:19:31 CET 2014

```Bob, what do you want to have happen when sum(1/a(n)) goes above 1? Your
multiplicand will go negative and the sum will start to drop. It's not
clear to me that it has a well-defined limit.

On Wed, Nov 19, 2014 at 10:26 AM, Bob Selcoe <rselcoe at entouchonline.net>
wrote:

> Hello Seqfans,
>
> Is there a way to calculate the following limit as n approaches infinity?
> This will be helpful in a comment I intend to add to A001097 (twin primes)
>
> Not sure about the most efficient or best way to notate, so here's what
>
> For a(n)>1, let a(n) = A007310 (numbers congruent to 1 or 5 mod 6): 5, 7,
> 11, 13, 17, 19, 23, 25....
>
> So a(1) = 5, a(2) = 7, a(3) = 11 etc.
>
> Calculate 1/a(1)  +  1/a(2)*(1 - 1/a(1))  +  1/a(3)*(1 - 1/a(1) - 1/a(2))
> +  1/a(4)*(1 - 1/a(1) - 1/a(2) - 1/a(3))...
>
> So 1/5 + 4/35 + 23/385 + 218/5005 +...
>
> If this is not easily solved, could someone please run a program to get an
> approximation?
>