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 A055557 Numbers n such that 3*R_n - 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 8
 2, 3, 4, 5, 6, 7, 8, 18, 40, 50, 60, 78, 101, 151, 319, 382, 784, 1732, 1918, 8855, 11245, 11960, 12130, 18533, 22718, 23365, 24253, 24549, 25324, 30178, 53718, 380976, 424861 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers n such that (10^n-7)/3 is prime. Sierpiński attributes the primes for n = 2,...,8 to A. Makowski. The history of the discovery of these numbers may be as follows: a(1)-a(7), Makowski; a(8)-a(18), Caldwell; a(19), Earls; a(20)-a(31), Kamada. (Corrections to this account will be welcomed.) Concerning certifying primes, see the references by Goldwasser et al., Atkin et al. and Morain. - Labos No more than 14 consecutive exponents can provide primes because for exponents 15k+2, 16k+9, 18k+12, 22k+21, a(n)'s are divisible by 31, 17, 19, 23 respectively. Here 7 of possible 14 is realized. - Labos Elemer, Jan 19 2005 (10^(15k+2)-7)/3 == 0 (mod 31). So 15k+2 isn't a term for k > 0. - Seiichi Manyama, Nov 05 2016 REFERENCES C. Caldwell, The near repdigit primes 333...331, J.Recreational Math. 21:4 (1989) 299-304. S. Goldwasser and J. Kilian, Almost All Primes Can Be Quickly Certified. in Proc. 18th STOC, 1986, pp. 316-329. W. Sierpiński, 200 Zadan z Elementarnej Teorii Liczb [200 Problems from the Elementary Theory of Numbers], Warszawa, 1964; Problem 88. LINKS A. Atkin and F. Morain, Elliptic Curves and Primality Proving, Math. Comp. 61:29-68, 1993. Makoto Kamada, Prime numbers of the form 33...331. Mathematics.StackExchange.com, 31,331,3331, 33331,333331,3333331,33333331 are prime F. Morain, Implementation of the Atkin-Goldwasser-Kilian Primality Testing Algorithm, INRIA Research Report, # 911, October 1988. Dave Rusin, Primes in exponential sequences [Broken link] Dave Rusin, Primes in exponential sequences [Cached copy] FORMULA a(n) = A055520(n) + 1. MATHEMATICA Do[ If[ PrimeQ[(10^n - 7)/3], Print[n]], {n, 50410}] One may run the prime certificate program as follows <True]}, {n, 1, 16}] (* Labos Elemer *) PROG (PARI) for(n=1, 2000, if(isprime((10^n-7)/3), print(n))) CROSSREFS Cf. A051200, A033175, A055520. Sequence in context: A250043 A260354 A114799 * A173577 A103205 A039127 Adjacent sequences:  A055554 A055555 A055556 * A055558 A055559 A055560 KEYWORD nonn AUTHOR Labos Elemer, Jul 10 2000 EXTENSIONS Corrected and extended by Jason Earls, Sep 22 2001 a(20)-a(31) were found by Makoto Kamada (see links for details). At present they correspond only to probable primes. a(32)-a(33) from Leonid Durman, Jan 09-10 2012 STATUS approved

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Last modified June 24 01:22 EDT 2018. Contains 311819 sequences. (Running on oeis4.)