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A120375 Integers k such that 2*5^k - 1 is prime. 5
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
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 June 14 08:28 EDT 2024. Contains 373393 sequences. (Running on oeis4.)