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A120375
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Integers k such that 2*5^k - 1 is prime.
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4
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4, 6, 16, 24, 30, 54, 96, 178, 274, 1332, 2766, 3060, 4204, 17736, 190062, 223536, 260400, 683080
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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Table of n, a(n) for n=1..18.
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FORMULA
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a(n) = 2*A002958(n).
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EXAMPLE
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a(1) = 4 since 2*5^4 - 1 = 1249 is the first prime.
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MAPLE
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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;
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MATHEMATICA
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Select[Range[0, 100], PrimeQ[2*5^# - 1] &] (* Robert Price, Mar 14 2015 *)
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PROG
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(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
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CROSSREFS
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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
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KEYWORD
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nonn,more
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AUTHOR
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Walter Kehowski, Jun 28 2006
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EXTENSIONS
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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
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STATUS
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approved
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