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A305702
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a(n) is the denominator of Sum_{primes p < n} 1/(n-p).
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2
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1, 1, 1, 2, 6, 12, 20, 10, 84, 840, 72, 630, 1320, 2772, 1560, 90090, 42, 240240, 1904, 46410, 95760, 639540, 5040, 9699690, 637560, 14316120, 92400, 176125950, 308880, 20078358300, 475020, 67690350, 7447440, 116925953760, 110880, 1002802450650, 318240, 122453100, 906665760, 165723680430
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OFFSET
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1,4
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COMMENTS
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The first n >= 2 for which a(n) <> A130492(n-1) is 32.
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LINKS
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EXAMPLE
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Sum_{primes p < 6} 1/(6-p) = 1/(6-2) + 1/(6-3) + 1/(6-5) = 19/12 so a(6) = 12.
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MAPLE
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N:= 100: # to get a(1)..a(N)
P:= select(isprime, [2, seq(i, i=3..N, 2)]):
seq(denom(add(1/(n-p), p=select(`<`, P, n))), n=1..N);
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PROG
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(PARI) a(n) = my(p=select(x->isprime(x), [1..n-1])); denominator(sum(k=1, #p, 1/(n-p[k]))); \\ Michel Marcus, Jun 09 2018
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CROSSREFS
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KEYWORD
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nonn,frac
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AUTHOR
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STATUS
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approved
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