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A099442
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Semiprimes of the form 2^k-k-1.
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4
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4, 26, 57, 247, 502, 4083, 1073741793, 4294967263, 8589934558, 70368744177617, 4503599627370443, 4611686018427387841, 18889465931478580854709, 75557863725914323419059, 77371252455336267181195177, 316912650057057350374175801245
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history;
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internal format)
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OFFSET
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1,1
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COMMENTS
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Semiprimes in A000295.
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LINKS
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Hugo Pfoertner, Table of n, a(n) for n = 1..31 (first 19 terms from Vincenzo Librandi)
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EXAMPLE
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a(4) = 247 because 247 = 13*19 = 2^8-8-1 = 2^A099441(4)-A099441(4)-1.
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MATHEMATICA
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Select[Table[2^n - n - 1, {n, 300}], PrimeOmega[#] == 2&] (* Vincenzo Librandi, Sep 21 2012 *)
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PROG
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(Python)
from sympy.ntheory.factor_ import primeomega
def ok(n): return primeomega(2**n-n-1) == 2
print([2**m-m-1 for m in range(2, 100) if ok(m)]) # Michael S. Branicky, Apr 26 2021
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CROSSREFS
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Cf. A099439, A099440, A099441.
Sequence in context: A102203 A219668 A273981 * A014450 A283573 A200058
Adjacent sequences: A099439 A099440 A099441 * A099443 A099444 A099445
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KEYWORD
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nonn
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AUTHOR
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Hugo Pfoertner, Oct 18 2004
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STATUS
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approved
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