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A002957
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Numbers n such that 2*10^n - 1 is prime.
(Formerly M0680)
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7
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1, 2, 3, 5, 7, 26, 27, 53, 147, 236, 248, 386, 401, 546, 785, 1325, 1755, 2906, 3020, 5407, 5697, 5969, 7517, 15749, 19233, 38232, 55347
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| Also numbers n such that 10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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REFERENCES
| H. Riesel, "Prime numbers and computer methods for factorization," Progress in Mathematics, Vol. 57, Birkhauser, Boston, 1985, Page 162.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
C. R. Zarnke and H. C. Williams, Computer determination of some large primes, pp. 563-570 in Proceedings of the Louisiana Conference on Combinatorics, Graph Theory and Computer Science. Vol. 2, edited R. C. Mullin et al., 1971.
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LINKS
| Makoto Kamada, Factorizations of 199...99
Index entries for primes involving repunits
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MATHEMATICA
| Do[ If[ PrimeQ[ 2*10^n - 1], Print[n] ], {n, 1, 15000} ]
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CROSSREFS
| Sequence in context: A088054 A085907 A024777 * A117135 A019372 A117299
Adjacent sequences: A002954 A002955 A002956 * A002958 A002959 A002960
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KEYWORD
| hard,nonn,more
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), Simon Plouffe (simon.plouffe(AT)gmail.com)
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EXTENSIONS
| Corrected and extended by Robert G. Wilson v (rgwv(AT)rgwv.com), Feb 02 2001.
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
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