login
Numbers n such that (10^n + 9^n)/19 is prime.
13

%I #27 Feb 17 2017 12:26:57

%S 7,67,73,1091,1483,10937

%N Numbers n such that (10^n + 9^n)/19 is prime.

%C The numbers n themselves (7, 67, 73, ...) are also prime.

%C a(7) > 10^5.

%t Select[ Prime[ Range[1, 100000] ], PrimeQ[ (10^# + 9^#)/19 ]& ]

%o (PARI) is(n)=isprime((10^n+9^n)/19) \\ _Charles R Greathouse IV_, Feb 17 2017

%Y Cf. A057175, A125956, A128339, A181141, A187819.

%K hard,nonn,more

%O 1,1

%A _Robert Price_, Feb 17 2013