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Integers n such that 2*11^n-1 is prime.
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%I #12 Nov 07 2015 09:19:06

%S 2,8,248,2474,2900,6600,24746,105704

%N Integers n such that 2*11^n-1 is prime.

%C 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.

%C a(9) > 2*10^5. - _Robert Price_, Nov 06 2015

%F a(n) = n-th integer k such that 2*11^k-1 is prime.

%e a(1)=2 since 2*11^2-1=241 is the first prime of this form.

%p 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;

%t Select[Range[0, 200000], PrimeQ[2*11^# - 1] &] (* _Robert Price_, Nov 06 2015 *)

%Y Cf. A000043, A000668, A002957, A002959, A003307, A079363, A055558.

%K more,nonn

%O 1,1

%A _Walter Kehowski_, Jun 28 2006

%E More terms from _Ryan Propper_, Jan 14 2008

%E a(7)-a(8) from _Robert Price_, Nov 06 2015