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

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Last modified July 24 13:42 EDT 2021. Contains 346273 sequences. (Running on oeis4.)