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 A092559 Numbers k such that 2^k + 1 is a semiprime. 15
 3, 5, 6, 7, 11, 12, 13, 17, 19, 20, 23, 28, 31, 32, 40, 43, 61, 64, 79, 92, 101, 104, 127, 128, 148, 167, 191, 199, 256, 313, 347, 356, 596, 692, 701, 1004, 1228, 1268, 1709, 2617, 3539, 3824, 5807, 10501, 10691, 11279, 12391, 14479, 42737, 83339, 95369, 117239 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Thanks to the recently found factor of F_14 (see A093179), we know that 16384 is not in the sequence. First unknown: 16768. - Don Reble, Mar 28 2010 The big prime factors for "5807" and all smaller entries have been proved prime; the rest (as far as I know) are probable primes. - Don Reble, Mar 28 2010 From Giuseppe Coppoletta, May 09 2017: (Start) As 3 divides 2^a(n) + 1 for any odd a(n), all odd terms are prime and they are exactly the Wagstaff numbers (A000978) or also the prime Jacobsthal indices (A107036). All terms from a(51) onwards refer to probabilistic primality tests for 2^a(n) + 1 (see Caldwell's link for the list of the largest certified Wagstaff primes). For the close relationship between this sequence and the Fermat numbers, see comments in A073936. The only difference is that here a term can be the square of a prime p, and by the Mihăilescu Theorem (also known as Catalan's conjecture, see link) that implies p = a(n) = 3. So, excluding a(1) = 3, they must coincide. As for A073936, after a(57), the values 267017, 269987, 374321, 986191, 4031399 and 4101572 are also terms, but there still remains the remote possibility of some gaps in between. In addition, 13347311 and 13372531 are also terms, but are possibly much further along in the numbering (see comments in A000978). (End). LINKS Giuseppe Coppoletta, Table of n, a(n) for n = 1..57 C. Caldwell's The Top Twenty Wagstaff primes. S. S. Wagstaff, Jr., The Cunningham Project. Eric W. Weisstein, MathWorld: Catalan's Conjecture. EXAMPLE 11 is a term because 2^11 + 1 = 3 * 683. 3 is a term because 2^3 + 1 = 3^2, while 10 is not a term because 2^10 + 1 = 5^2 * 41. MATHEMATICA Select[Range@ 200, PrimeOmega[2^# + 1] == 2 &] (* Michael De Vlieger, May 09 2017 *) PROG (PARI) isok(n) = bigomega(2^n+1) == 2; \\ Michel Marcus, Oct 05 2013 CROSSREFS Cf. A085724, A092558, A092561, A092562, A000978, A107036, A066263. Cf. A073936. - R. J. Mathar, Sep 08 2008 Sequence in context: A129802 A023854 A324511 * A242076 A064728 A046839 Adjacent sequences:  A092556 A092557 A092558 * A092560 A092561 A092562 KEYWORD nonn AUTHOR Zak Seidov, Feb 27 2004 EXTENSIONS More terms from Cunningham project, Mar 23 2004 More terms from Don Reble, Mar 28 2010 a(49)-a(52) from Giuseppe Coppoletta, May 08 2017 STATUS approved

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Last modified February 18 02:57 EST 2020. Contains 332006 sequences. (Running on oeis4.)