A286959 - OEIS

%I #26 Jul 24 2021 02:25:27

%S 11,17,197,317,347,431,977,1949,1991,2913,6317,9725,36599

%N Positive numbers k such that (10^(k+3)*3277 + 3167)/9 is prime.

%C Or '364'||...'1'...||'463' in decimal form is prime ('1' concatenated k times to which the prefix '364' and the suffix '463' are concatenated once).

%C a(1)..a(8) themselves are primes.

%e 11 is a term as 36411111111111463 is prime (as a string, it consists of '1' concatenated 11 times to which the prefix '364' and the suffix '463' are concatenated once).

%t ParallelMap[ If[ PrimeQ[(10^(#+3)*3277+3167)/9], #, Nothing]&, Range[3000]]

%o (PARI) is(n)=ispseudoprime((10^(n+3)*3277+3167)/9) \\ _Charles R Greathouse IV_, Jun 13 2017

%K nonn,hard,more,base

%O 1,1

%A _Mikk Heidemaa_, May 17 2017

%E a(13) from _Giovanni Resta_, May 25 2017