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 A003307 Numbers k such that 2*3^k - 1 is prime. (Formerly M0823) 30
 1, 2, 3, 7, 8, 12, 20, 23, 27, 35, 56, 62, 68, 131, 222, 384, 387, 579, 644, 1772, 3751, 5270, 6335, 8544, 9204, 12312, 18806, 21114, 49340, 75551, 90012, 128295, 143552, 147488, 1010743, 1063844, 1360104 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES R. K. Guy, Unsolved Problems in Theory of Numbers, Section A3. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Kevin A. Broughan and Qizhi Zhou, On the Ratio of the Sum of Divisors and Euler's Totient Function II, Journal of Integer Sequences, Vol. 17 (2014), Article 14.9.2. Steven Harvey, Williams Primes W. Keller, J. Richstein, Solutions of the congruence a^(p-1) == 1 (mod p^r), Math. Comp. 74 (2005), 927-936. H. C. Williams, The primality of certain integers of the form 2Ar^n - 1, Acta Arith. 39 (1981), 7-17. H. C. Williams and C. R. Zarnke, Some prime numbers of the forms 2*3^n+1 and 2*3^n-1, Math. Comp., 26 (1972), 995-998. PROG (PARI) for(n=1, 1e4, if(ispseudoprime(2*3^n-1), print1(n", "))) \\ Charles R Greathouse IV, Jul 16 2011 CROSSREFS Cf. A002235, A046865, A079906, A046866, A001771, A005541, A056725, A046867, A079907. Cf. A079363 (primes of the form 2*3^n-1), A003306 (n such that 2*3^n+1 is prime). Sequence in context: A184860 A242655 A145489 * A105601 A199971 A033082 Adjacent sequences:  A003304 A003305 A003306 * A003308 A003309 A003310 KEYWORD nonn,hard,nice AUTHOR EXTENSIONS More terms from Douglas Burke (dburke(AT)nevada.edu) More terms from T. D. Noe, Aug 24 2005 Corrected and extended by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 05 2008 a(35) from Borys Jaworski, Sep 02 2011 a(36) from Borys Jaworski, Feb 13 2012 a(37) from Jeppe Stig Nielsen, Sep 28 2018 STATUS approved

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Last modified January 22 13:36 EST 2020. Contains 331149 sequences. (Running on oeis4.)