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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.)