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A181904
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a(n) = 2*(4^n - 1) / A027760(n)
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1
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3, 5, 63, 17, 1023, 195, 16383, 4369, 262143, 31775, 4194303, 12291, 67108863, 89478485, 1073741823, 16843009, 17179869183, 172229265, 274877906943, 6663706835, 4398046511103, 254959218035, 70368744177663, 206208774147, 1125899906842623, 1501199875790165
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OFFSET
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1,1
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COMMENTS
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The Clausen numbers A027760 divide 2*(4^n - 1).
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LINKS
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MATHEMATICA
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a[n_] := 2*(4^n-1)/Times @@ Select[Divisors[n]+1, PrimeQ]; Table[a[n], {n, 1, 26}] (* Jean-François Alcover, Jul 29 2013 *)
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PROG
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(Sage)
S = divisors(n)
S = map(lambda i: i+1, S)
S = filter(lambda i: is_prime(i), S)
return mul(i for i in 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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STATUS
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approved
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