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 A120378 Integers n such that 2*11^n-1 is prime. 5
 2, 8, 248, 2474, 2900, 6600, 24746, 105704 (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, 241 is 181 in base 12. a(9) > 2*10^5. - Robert Price, Nov 06 2015 LINKS FORMULA a(n) = n-th integer k such that 2*11^k-1 is prime. EXAMPLE a(1)=2 since 2*11^2-1=241 is the first prime of this form. MAPLE for w to 1 do for k from 1 to 2000 do n:=2*11^k-1; if isprime(n) then printf("%d, %d", k, n) fi od od; MATHEMATICA Select[Range[0, 200000], PrimeQ[2*11^# - 1] &] (* Robert Price, Nov 06 2015 *) CROSSREFS Cf. A000043, A000668, A002957, A002959, A003307, A079363, A055558. Sequence in context: A125264 A028985 A067583 * A012345 A012341 A299374 Adjacent sequences:  A120375 A120376 A120377 * A120379 A120380 A120381 KEYWORD more,nonn AUTHOR Walter Kehowski, Jun 28 2006 EXTENSIONS More terms from Ryan Propper, Jan 14 2008 a(7)-a(8) from Robert Price, Nov 06 2015 STATUS approved

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Last modified April 11 00:03 EDT 2021. Contains 342877 sequences. (Running on oeis4.)