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A117673
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a(n) is the least k such that k*2*prime(n) + 1 is prime.
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6
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1, 1, 1, 2, 1, 2, 3, 5, 1, 1, 5, 2, 1, 2, 3, 1, 6, 3, 2, 4, 2, 2, 1, 1, 2, 3, 3, 3, 5, 1, 2, 1, 3, 2, 4, 3, 5, 2, 7, 1, 1, 3, 1, 2, 9, 2, 5, 6, 12, 6, 1, 1, 3, 1, 3, 3, 4, 3, 2, 1, 3, 1, 2, 3, 3, 13, 3, 5, 3, 5, 7, 1, 3, 2, 6, 6, 12, 3, 4, 2, 1, 5, 1, 2, 5, 1, 4, 15, 3, 6, 3, 4, 2, 1, 2, 3, 1, 16, 5, 9
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OFFSET
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1,4
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COMMENTS
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LINKS
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EXAMPLE
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a(8)=5 because 2*prime(8)=38 and 5*38 + 1 is prime.
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MATHEMATICA
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Table[k := 1; While[ ! PrimeQ[2*k*Prime[n] + 1], k++ ]; k, {n, 1, 120}] (* Stefan Steinerberger, May 01 2006 *)
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PROG
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(PARI) a(n) = {my(p=prime(n), k=1); while (!isprime(2*k*p+1), k++); k; } \\ Michel Marcus, Feb 12 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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