This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 (list; graph; refs; listen; history; text; internal format)
 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. 563-570 in Proceedings of the Louisiana Conference on Combinatorics, Graph Theory and Computer Science. Vol. 2, edited R. C. Mullin et al., 1971. LINKS Makoto Kamada, Prime numbers of the form 199...99. MATHEMATICA Do[ If[ PrimeQ[ 2*10^n - 1], Print[n] ], {n, 1, 15000} ] PROG (PARI) for(n=1, 10^5, if(ispseudoprime(2*10^n-1), 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 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 19 11:12 EDT 2019. Contains 323390 sequences. (Running on oeis4.)