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%I
%S 7,43,55987,7369130657357778596659,
%T 3546245297457217493590449191748546458005595187661976371
%N Primes of the form (6^m-1)/5.
%C Prime repunits in base 6 whose representation consists of m 1's. The exponents m are in A004062. a(5) and a(6) have 55 and 99 decimal digits, respectively.
%H Vincenzo Librandi, <a href="/A165210/b165210.txt">Table of n, a(n) for n = 1..9</a>
%F a(n) = (6^A004062(n)-1)/5.
%e a(2) = (6^A004062(2)-1)/5 = (6^3-1)/5 = 215/5 = 43, which is 111 (base 6).
%t Select[Table[(6^n-1)/5, {n,0,2000}], PrimeQ] (* Vincenzo Librandi, Dec 09 2011 *)
%o [a: n in [1..200] | IsPrime(a) where a is (6^n-1) div 5 ]; // (MAGMA) Vincenzo Librandi, Dec 09 2011
%Y Cf. A004062, A003464, A085104, A000668, A076481, A086122, A102170, A004022.
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Sep 07 2009
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