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A326577
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a(n) = (2*n - 1) / A326478(2*n - 1).
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5
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1, 3, 5, 7, 3, 11, 13, 3, 17, 19, 3, 23, 5, 3, 29, 31, 3, 1, 37, 3, 41, 43, 15, 47, 7, 3, 53, 1, 3, 59, 61, 3, 5, 67, 3, 71, 73, 3, 1, 79, 3, 83, 5, 3, 89, 7, 3, 1, 97, 3, 101, 103, 15, 107, 109, 3, 113, 1, 3, 1, 11, 3, 5, 127, 3, 131, 7, 3, 137, 139, 3, 1, 5
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OFFSET
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1,2
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LINKS
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FORMULA
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If 2*n - 1 is prime then a(n) is prime.
a(n) = gcd((2*n-1)*N(2*n-2), D(2*n-2)), with N(k)/D(k) = B(k) the k-th Bernoulli number.
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MAPLE
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db := n -> denom(bernoulli(n)): nb := n -> numer(bernoulli(n)):
a := n -> igcd(db(2*n-2), (2*n-1)*nb(2*n-2)): seq(a(n), n=1..72);
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PROG
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(PARI) f(n) = n*denominator(n*bernfrac(n-1))/denominator(bernfrac(n-1)); \\ A326478
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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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