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A092561
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"Mersenne" semiprimes, semiprimes of the form 2^k-1.
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8
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15, 511, 2047, 8388607, 137438953471, 2199023255551, 562949953421311, 576460752303423487, 147573952589676412927, 9671406556917033397649407, 158456325028528675187087900671, 2535301200456458802993406410751
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OFFSET
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1,1
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LINKS
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Amiram Eldar, Table of n, a(n) for n = 1..42
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FORMULA
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a(n) = A000225(A085724(n)). - Amiram Eldar, Jun 18 2022
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EXAMPLE
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2047 is a member because 2047 = 2^11 - 1 = 23*89.
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MATHEMATICA
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a := Select[Range[2, 120], ! PrimeQ[2^# - 1] && Length[Divisors[2^# - 1]] <= 4 &]; 2^a - 1 (* Stefan Steinerberger, Apr 12 2006 *)
Select[2^Range[0, 110]-1, PrimeOmega[#] == 2&] (* Harvey P. Dale, Feb 22 2013 *)
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CROSSREFS
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Corresponding k: A085724.
Cf. A000225, A092558, A092559, A092562.
Sequence in context: A218188 A218365 A203326 * A218312 A218280 A218632
Adjacent sequences: A092558 A092559 A092560 * A092562 A092563 A092564
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KEYWORD
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nonn
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AUTHOR
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Zak Seidov, Feb 27 2004
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EXTENSIONS
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One more term from Stefan Steinerberger, Apr 12 2006
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STATUS
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approved
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