

A002957


Numbers n such that 2*10^n  1 is prime.
(Formerly M0680)


11



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
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OFFSET

1,2


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.
Serge Batalov discovered that 1059002 belongs to this sequence but may be not the next term.  Max Alekseyev, Sep 30 2013
a(28) > 410000 (from Kamada data).  Robert Price, Oct 19 2014


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. 563570 in Proceedings of the Louisiana Conference on Combinatorics, Graph Theory and Computer Science. Vol. 2, edited R. C. Mullin et al., 1971.


LINKS

Table of n, a(n) for n=1..27.
Makoto Kamada, Prime numbers of the form 199...99.
Index entries for primes involving repunits


MATHEMATICA

Do[ If[ PrimeQ[ 2*10^n  1], Print[n] ], {n, 1, 15000} ]


PROG

(PARI) for(n=1, 10^5, if(ispseudoprime(2*10^n1), print1(n, ", "))) \\ Felix FrÃ¶hlich, Jun 23 2014


CROSSREFS

Sequence in context: A249509 A085907 A024777 * A273726 A211660 A215155
Adjacent sequences: A002954 A002955 A002956 * A002958 A002959 A002960


KEYWORD

hard,nonn,more


AUTHOR

N. J. A. Sloane, Simon Plouffe


EXTENSIONS

Corrected and extended by Robert G. Wilson v, Feb 02 2001
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008


STATUS

approved



