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A002236
Numbers k such that 9*2^k - 1 is prime.
(Formerly M2634 N1045)
9
1, 3, 7, 13, 15, 21, 43, 63, 99, 109, 159, 211, 309, 343, 415, 469, 781, 871, 939, 1551, 3115, 3349, 5589, 5815, 5893, 7939, 8007, 11547, 12495, 22555, 23647, 35647, 83415, 103059, 184999, 275859, 384243, 484975, 503893, 828709, 1010277, 1419855, 1481821
OFFSET
1,2
COMMENTS
Even exponents can give at most semiprimes (see A181490). - Jeppe Stig Nielsen, Jun 08 2023
REFERENCES
H. Riesel, "Prime numbers and computer methods for factorization," Progress in Mathematics, Vol. 57, Birkhauser, Boston, 1985, Chap. 4, see pp. 381-384.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
MATHEMATICA
b=9; i=0; Table[While[i++; cp=b*2^i-1; !PrimeQ[cp]]; i, {j, 1, 22}] (* Lei Zhou, Nov 08 2013 *)
Select[Range[3400], PrimeQ[9*2^#-1]&] (* The program generates the first 22 terms of the sequence. To generate more, increase the Range constant, but the program may take a long time to run. *) (* Harvey P. Dale, Sep 01 2020 *)
PROG
(PARI) is(n)=ispseudoprime(9*2^n-1) \\ Charles R Greathouse IV, Feb 17 2017
CROSSREFS
Cf. A050524.
Sequence in context: A092734 A209839 A192854 * A255682 A080565 A164344
KEYWORD
hard,nonn,nice
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(42)-a(43) communicated by Jeppe Stig Nielsen, Jun 08 2023
STATUS
approved