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Consider the strings $(b,b+1,b+2,...,b+l)$ of consecutive natural numbers, all less than some fixed natural number $n > b+l$. Is there a way to find the longest string (length of a string $= l+1$) with $\gcd(b+i,n) > 1$ for all $0\le i \le l$?

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Search " Jacobsthal function" on Google or the OEIS. –  Charles Oct 11 '12 at 18:32

1 Answer 1

Arrange arithmetic sequences with distances given by the prime factors of $n$ so as to cover as many consecutive numbers as possible; for instance, if the prime factors are $2$, $3$, $5$ and $7$, this could be

      2 3 2 7 2 5 2 3 2
              3

I don't know whether there's a systematic way of doing this, but it seems that just using them from the smallest to largest whenever a new one is required to fill a gap might be optimal. Then by the Chinese remainder theorem, using the associated algorithm you can find the residue mod $n$ where this sequence is realized.

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To expand: if $n=(2)(3)(5)(7)=210$, then you get length 9 by choosing $b$ so $b$ is a multiple of 2, $b+1$ is a multiple of 3, $b+3$ is a multiple of 7, and $b+5$ is amultiple of 5; the algorithm in the link finds such a number $b$. –  Gerry Myerson Oct 12 '12 at 5:54
    
Thanks for the clarity Gerry. I have a related question (to the OP) which I really wanted answered, and I think Charles' hint has given me an answer. The question is "Does (l(n)) / n tend to 0 as n tends to infinity?" The motive is that it would give me some interesting "nested dense" sets in R. Well, it is interesting to me because I am exploring the Baire Category theorem. Anyway, after searching the Jacobsthal function on google I found that l(n) < (log(n))^2 and since (log(n))^2 / n tends to 0 as n tends to infinity, my question is answered. Now to create interesting nested dense sets... –  Adam Rubinson Oct 12 '12 at 10:46
    
Thanks for your answers btw –  Adam Rubinson Oct 12 '12 at 10:48
    
@Adam: You might be interested in this question and the paper linked to in my answer. –  joriki Oct 12 '12 at 10:57

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