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 A103961 Least k such that 2*n*k - 1 is a prime. 2
 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 4, 3, 1, 1, 2, 2, 1, 2, 1, 1, 3, 1, 3, 2, 1, 3, 3, 1, 1, 2, 2, 1, 2, 1, 1, 2, 3, 1, 2, 1, 3, 3, 1, 4, 3, 2, 1, 2, 1, 1, 2, 1, 1, 2, 1, 3, 3, 2, 4, 5, 2, 1, 3, 1, 3, 2, 1, 1, 2, 3, 7, 3, 1, 1, 2, 2, 1, 3, 4, 1, 2, 1, 3, 5, 1, 7, 8, 1, 1, 2, 3, 3, 2, 1, 1, 3, 1, 1, 5, 5, 3, 5, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Question: Is the sequence unbounded (like A016014)? - Dmitry Kamenetsky, Oct 26 2016 Answer: Yes. Essentially the same argument works. To get n such that a(n) > K, take distinct odd primes p_k, k=1..K with p_k not dividing k, and take n such that n == (2*k)^(-1) mod p_k and 2*k*n-1 > p_k for k=1..K. - Robert Israel, Oct 27 2016 LINKS Dmitry Kamenetsky, Table of n, a(n) for n = 1..10000 EXAMPLE 2*1*2-1 = 3, so a(1) = 2; 2*5*2-1 = 19, so a(5) = 2. MATHEMATICA Do[k = 1; cp = n*k - 1; While[ ! PrimeQ[cp], k++; cp = n*k - 1]; Print[k], {n, 2, 400, 2}] PROG (PARI) a(n) = {my(k=1); while (!isprime(2*n*k-1), k++); k; } \\ Michel Marcus, Oct 27 2016 CROSSREFS Cf. A016014. Sequence in context: A106035 A293811 A105141 * A220464 A215975 A071891 Adjacent sequences:  A103958 A103959 A103960 * A103962 A103963 A103964 KEYWORD easy,nonn AUTHOR Lei Zhou, Feb 23 2005 STATUS approved

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Last modified April 24 22:26 EDT 2019. Contains 322446 sequences. (Running on oeis4.)