A126716 - OEIS

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A126716

Least i such that prime(i)*2^(10^n) -1 is prime.

%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