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 A120375 Integers k such that 2*5^k - 1 is prime. 4
 4, 6, 16, 24, 30, 54, 96, 178, 274, 1332, 2766, 3060, 4204, 17736, 190062, 223536, 260400, 683080 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS See comments for A057472. Examined in base 12, all n must be even and all primes must be 1-primes. For example, 1249 is 881 in base 12. a(16) > 2*10^5. - Robert Price, Mar 14 2015 LINKS FORMULA a(n) = 2*A002958(n). EXAMPLE a(1) = 4 since 2*5^4 - 1 = 1249 is the first prime. MAPLE for w to 1 do for k from 1 to 2000 do n:=2*5^k-1; if isprime(n) then printf("%d, %d ", k, n) fi od od; MATHEMATICA Select[Range[0, 100], PrimeQ[2*5^# - 1] &] (* Robert Price, Mar 14 2015 *) PROG (PARI) isok(k) = ispseudoprime(2*5^k-1); \\ Altug Alkan, Sep 22 2018 (MAGMA) [n: n in [0..2800] |IsPrime(2*5^n - 1)]; // Vincenzo Librandi, Sep 23 2018 CROSSREFS Integers k such that 2*b^k - 1 is prime: A090748 (b=2), A003307 (b=3), this sequence (b=5), A057472 (b=6), A002959 (b=7), A002957 (b=10), A120378 (b=11). Primes of the form 2*b^k - 1: A000668 (b=2), A079363 (b=3), A120376 (b=5), A158795 (b=7), A055558 (b=10), A120377 (b=11). Cf. also A000043, A002958. Sequence in context: A097970 A044858 A323043 * A025618 A133572 A121852 Adjacent sequences:  A120372 A120373 A120374 * A120376 A120377 A120378 KEYWORD nonn,more AUTHOR Walter Kehowski, Jun 28 2006 EXTENSIONS More terms from Ryan Propper, Mar 28 2007 a(14) from Herman Jamke (hermanjamke(AT)fastmail.fm), May 02 2007 a(15) from Robert Price, Mar 14 2015 a(16)-a(18) from Jorge Coveiro and Tyler NeSmith, Jun 14 2020 STATUS approved

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Last modified November 24 07:14 EST 2020. Contains 338607 sequences. (Running on oeis4.)