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A071558
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Smallest k such that n*k + 1 and n*k - 1 are twin primes.
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16
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4, 2, 2, 1, 6, 1, 6, 9, 2, 3, 18, 1, 24, 3, 2, 12, 6, 1, 12, 3, 2, 9, 6, 3, 6, 12, 4, 15, 12, 1, 42, 6, 6, 3, 12, 2, 54, 6, 8, 6, 30, 1, 24, 15, 4, 3, 6, 4, 18, 3, 2, 6, 120, 2, 12, 48, 4, 6, 18, 1, 258, 21, 14, 3, 30, 3, 24, 15, 2, 6, 18, 1, 84, 27, 2, 3, 6, 4, 132, 3, 10, 15, 54, 5, 12, 12
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OFFSET
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1,1
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COMMENTS
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Conjecture: a(n) < sqrt(n)*log(n) for all n > 17261. This has been verified for n up to 3*10^7. It implies the inequality a(n) < n for each n > 127. - Zhi-Wei Sun, Jan 07 2013
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LINKS
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MATHEMATICA
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Table[k=1; While[!And@@PrimeQ[n*k+{1, -1}], k++]; k, {n, 86}] (* Jayanta Basu, May 26 2013 *)
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PROG
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(PARI) for(n=1, 100, s=1; while(isprime(s*n+1)*isprime(n*s-1)==0, s++); print1(s, ", "))
(Haskell)
a071558 n = head [k | k <- [1..], let x = k * n,
a010051' (x - 1) == 1, a010051' (x + 1) == 1]
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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