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A261625
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Number of primes p <= n such that (p-1)*n+1 is prime.
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1
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0, 1, 1, 1, 1, 2, 2, 1, 2, 3, 2, 2, 4, 1, 4, 3, 1, 5, 2, 2, 5, 4, 3, 3, 4, 3, 5, 6, 3, 5, 3, 2, 6, 5, 5, 5, 3, 2, 5, 6, 3, 4, 6, 2, 7, 9, 2, 5, 5, 3, 9, 7, 1, 5, 7, 5, 5, 8, 2, 8, 7, 3, 8, 7, 5, 7, 6, 3, 6, 9, 5, 9, 7, 4, 6, 8, 3, 8, 9, 3
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OFFSET
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1,6
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COMMENTS
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Conjecture: a(n) > 0 for all n > 1.
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LINKS
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EXAMPLE
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a(53) = 1 since 3 and (3-1)*53+1 = 107 are both prime.
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MATHEMATICA
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Do[r=0; Do[If[PrimeQ[(Prime[k]-1)n+1], r=r+1], {k, 1, PrimePi[n]}]; Print[n, " ", r]; Continue, {n, 1, 80}]
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PROG
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(PARI) a(n) = my(nb=0); forprime(p=2, n, if (isprime((p-1)*n+1), nb++)); nb; \\ Michel Marcus, Aug 27 2015
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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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