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%I #6 Sep 15 2019 07:55:48
%S 2,3,51,657,6255
%N Least i such that prime(i)*2^(10^n) -1 is prime.
%e 3*2^(10^0)-1=5 prime so a(0)=2 since 3 is prime(2).
%e 3*2^(10^1)-1=3071=37*83 composite, 5*2^(10^1)-1=5119 prime so a(1)=3, since 5 is prime(3).
%o (PARI) a(n) = my(i=2); while (!isprime(prime(i)*2^(10^n) -1), i++); i; \\ _Michel Marcus_, Sep 15 2019
%K nonn,more
%O 0,1
%A _Pierre CAMI_, Feb 13 2007
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