%I

%S 1,3,4,7,132,253

%N Numbers n such that 2*7^n - 3 is prime.

%t Do[ If[ PrimeQ[ 2*7^n - 3 ], Print[n] ], {n, 1, 6000} ]

%o (PARI) is(n)=ispseudoprime(2*7^n-3) \\ _Charles R Greathouse IV_, Jun 13 2017

%K nonn

%O 1,2

%A _Robert G. Wilson v_, Feb 12 2001