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A099441 Numbers n such that A000295(n) = 2^n-n-1 is a semiprime. 3

%I

%S 3,5,6,8,9,12,30,32,33,46,52,62,74,76,86,98,130,134,154,228,230,242,

%T 256,266,346,352,382,412,428,474,488

%N Numbers n such that A000295(n) = 2^n-n-1 is a semiprime.

%C A candidate for the next term is a(32) = 634. 2^634-635 is composite with 191 decimal digits and unknown factorization. - _Hugo Pfoertner_, Aug 13 2007

%H Dario Alpern, <a href="http://www.alpertron.com.ar/ECM.HTM">Factorization using the Elliptic Curve Method.</a>

%H <a href="http://factordb.com/index.php?query=2%5E634-635">Status of 2^634-635 in factordb.com</a>.

%e a(3) = 6 because 2^6-6-1 = 57 = 3*19 is a semiprime.

%Y Cf. A000295 2^n-n-1 (column 2 of the Eulerian numbers), A099439 2^n-n-1 is prime, A099440 primes in A000295, A099442 semiprimes in A000295.

%K hard,more,nonn

%O 1,1

%A _Hugo Pfoertner_, Oct 18 2004

%E More terms from _Hugo Pfoertner_, Aug 13 2007

%E a(24) corrected by _Hugo Pfoertner_, Sep 07 2017

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Last modified May 25 03:39 EDT 2022. Contains 354047 sequences. (Running on oeis4.)