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A086668
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Number of divisors d of n such that 2d+1 is a prime.
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4
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1, 2, 2, 2, 2, 4, 1, 3, 3, 3, 2, 4, 1, 3, 4, 3, 1, 6, 1, 4, 3, 3, 2, 5, 2, 3, 3, 3, 2, 7, 1, 3, 4, 2, 3, 7, 1, 2, 3, 5, 2, 6, 1, 4, 5, 3, 1, 6, 1, 4, 3, 3, 2, 7, 3, 5, 2, 3, 1, 8, 1, 2, 5, 3, 3, 6, 1, 3, 4, 5, 1, 8, 1, 3, 5, 2, 2, 7, 1, 5, 4, 3, 2, 6, 2, 3, 3, 5, 2, 10, 1, 3, 2, 2, 3, 7, 1, 4, 6, 5
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OFFSET
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1,2
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COMMENTS
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Number of terms of A005097 that divide n.
For all n >= 1, a(n) > A156660(n). Specifically, a(p) = 2 for all p in A005384 (Sophie Germain primes), although 2's occur in other positions as well.
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LINKS
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FORMULA
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(End)
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EXAMPLE
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10 has divisors 1,2,5 and 10 of which 2.1+1, 2.2+1 and 2.5+1 are prime, so a(10)=3
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MATHEMATICA
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Table[Count[Divisors[n], _?(PrimeQ[2#+1]&)], {n, 100}] (* Harvey P. Dale, Apr 29 2015 *)
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PROG
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(PARI) for (n=2, 100, s=0; fordiv(i=n, i, s+=isprime(2*i+1)); print1(", "s))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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